Keeping consistent between UI and API about combined commit status state and fix some bugs (#34562)

Extract from #34531 

## Move Commit status state to a standalone package

Move the state from `structs` to `commitstatus` package. It also
introduce `CommitStatusStates` so that the combine function could be
used from UI and API logic.

## Combined commit status Changed

This PR will follow Github's combined commit status. Before this PR,
every commit status could be a combined one.
According to
https://docs.github.com/en/rest/commits/statuses?apiVersion=2022-11-28#get-the-combined-status-for-a-specific-reference
> Additionally, a combined state is returned. The state is one of:
> failure if any of the contexts report as error or failure
> pending if there are no statuses or a context is pending
> success if the latest status for all contexts is success

This PR will follow that rule and remove the `NoBetterThan` logic. This
also fixes the inconsistent between UI and API. In the API convert
package, it has implemented this which is different from the UI. It also
fixed the missing `URL` and `CommitURL` in the API.

## `CalcCommitStatus` return nil if there is no commit statuses

The behavior of `CalcCommitStatus` is changed. If the parameter commit
statuses is empty, it will return nil. The reference places should check
the returned value themselves.

templates/swagger/v1_json.tmpl

@@ -21030,7 +21030,17 @@
           "x-go-name": "SHA"
         },
         "state": {
-          "$ref": "#/definitions/CommitStatusState"
+          "type": "string",
+          "enum": [
+            "pending",
+            "success",
+            "error",
+            "failure",
+            "warning",
+            "skipped"
+          ],
+          "x-go-enum-desc": "pending CommitStatusPending  CommitStatusPending is for when the CommitStatus is Pending\nsuccess CommitStatusSuccess  CommitStatusSuccess is for when the CommitStatus is Success\nerror CommitStatusError  CommitStatusError is for when the CommitStatus is Error\nfailure CommitStatusFailure  CommitStatusFailure is for when the CommitStatus is Failure\nwarning CommitStatusWarning  CommitStatusWarning is for when the CommitStatus is Warning\nskipped CommitStatusSkipped  CommitStatusSkipped is for when CommitStatus is Skipped",
+          "x-go-name": "State"
         },
         "statuses": {
           "type": "array",
@@ -21258,7 +21268,17 @@
           "x-go-name": "ID"
         },
         "status": {
-          "$ref": "#/definitions/CommitStatusState"
+          "type": "string",
+          "enum": [
+            "pending",
+            "success",
+            "error",
+            "failure",
+            "warning",
+            "skipped"
+          ],
+          "x-go-enum-desc": "pending CommitStatusPending  CommitStatusPending is for when the CommitStatus is Pending\nsuccess CommitStatusSuccess  CommitStatusSuccess is for when the CommitStatus is Success\nerror CommitStatusError  CommitStatusError is for when the CommitStatus is Error\nfailure CommitStatusFailure  CommitStatusFailure is for when the CommitStatus is Failure\nwarning CommitStatusWarning  CommitStatusWarning is for when the CommitStatus is Warning\nskipped CommitStatusSkipped  CommitStatusSkipped is for when CommitStatus is Skipped",
+          "x-go-name": "State"
         },
         "target_url": {
           "type": "string",
@@ -21276,11 +21296,6 @@
       },
       "x-go-package": "code.gitea.io/gitea/modules/structs"
     },
-    "CommitStatusState": {
-      "description": "CommitStatusState holds the state of a CommitStatus\nIt can be \"pending\", \"success\", \"error\" and \"failure\"",
-      "type": "string",
-      "x-go-package": "code.gitea.io/gitea/modules/structs"
-    },
     "CommitUser": {
       "type": "object",
       "title": "CommitUser contains information of a user in the context of a commit.",
@@ -22329,7 +22344,17 @@
           "x-go-name": "Description"
         },
         "state": {
-          "$ref": "#/definitions/CommitStatusState"
+          "type": "string",
+          "enum": [
+            "pending",
+            "success",
+            "error",
+            "failure",
+            "warning",
+            "skipped"
+          ],
+          "x-go-enum-desc": "pending CommitStatusPending  CommitStatusPending is for when the CommitStatus is Pending\nsuccess CommitStatusSuccess  CommitStatusSuccess is for when the CommitStatus is Success\nerror CommitStatusError  CommitStatusError is for when the CommitStatus is Error\nfailure CommitStatusFailure  CommitStatusFailure is for when the CommitStatus is Failure\nwarning CommitStatusWarning  CommitStatusWarning is for when the CommitStatus is Warning\nskipped CommitStatusSkipped  CommitStatusSkipped is for when CommitStatus is Skipped",
+          "x-go-name": "State"
         },
         "target_url": {
           "type": "string",