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.

routers/web/repo/commit.go

@@ -457,6 +457,9 @@ func processGitCommits(ctx *context.Context, gitCommits []*git.Commit) ([]*git_m
 	}
 	if !ctx.Repo.CanRead(unit_model.TypeActions) {
 		for _, commit := range commits {
+			if commit.Status == nil {
+				continue
+			}
 			commit.Status.HideActionsURL(ctx)
 			git_model.CommitStatusesHideActionsURL(ctx, commit.Statuses)
 		}