Sign merges, CRUD, Wiki and Repository initialisation with gpg key (#7631)

This PR fixes #7598 by providing a configurable way of signing commits across the Gitea instance. Per repository configurability and import/generation of trusted secure keys is not provided by this PR - from a security PoV that's probably impossible to do properly. Similarly web-signing, that is asking the user to sign something, is not implemented - this could be done at a later stage however.

## Features
- [x] If commit.gpgsign is set in .gitconfig sign commits and files created through repofiles. (merges should already have been signed.)
- [x] Verify commits signed with the default gpg as valid
- [x] Signer, Committer and Author can all be different
    - [x] Allow signer to be arbitrarily different - We still require the key to have an activated email on Gitea. A more complete implementation would be to use a keyserver and mark external-or-unactivated with an "unknown" trust level icon.
- [x] Add a signing-key.gpg endpoint to get the default gpg pub key if available
    - Rather than add a fake web-flow user I've added this as an endpoint on /api/v1/signing-key.gpg
    - [x] Try to match the default key with a user on gitea - this is done at verification time
- [x] Make things configurable?
    - app.ini configuration done
    - [x] when checking commits are signed need to check if they're actually verifiable too
- [x] Add documentation

I have decided that adjusting the docker to create a default gpg key is not the correct thing to do and therefore have not implemented this.

templates/swagger/v1_json.tmpl

@@ -5140,6 +5140,42 @@
         }
       }
     },
+    "/repos/{owner}/{repo}/signing-key.gpg": {
+      "get": {
+        "produces": [
+          "text/plain"
+        ],
+        "tags": [
+          "repository"
+        ],
+        "summary": "Get signing-key.gpg for given repository",
+        "operationId": "repoSigningKey",
+        "parameters": [
+          {
+            "type": "string",
+            "description": "owner of the repo",
+            "name": "owner",
+            "in": "path",
+            "required": true
+          },
+          {
+            "type": "string",
+            "description": "name of the repo",
+            "name": "repo",
+            "in": "path",
+            "required": true
+          }
+        ],
+        "responses": {
+          "200": {
+            "description": "GPG armored public key",
+            "schema": {
+              "type": "string"
+            }
+          }
+        }
+      }
+    },
     "/repos/{owner}/{repo}/stargazers": {
       "get": {
         "produces": [
@@ -5691,6 +5727,26 @@
         }
       }
     },
+    "/signing-key.gpg": {
+      "get": {
+        "produces": [
+          "text/plain"
+        ],
+        "tags": [
+          "miscellaneous"
+        ],
+        "summary": "Get default signing-key.gpg",
+        "operationId": "getSigningKey",
+        "responses": {
+          "200": {
+            "description": "GPG armored public key",
+            "schema": {
+              "type": "string"
+            }
+          }
+        }
+      }
+    },
     "/teams/{id}": {
       "get": {
         "produces": [
@@ -9525,6 +9581,9 @@
           "type": "string",
           "x-go-name": "Signature"
         },
+        "signer": {
+          "$ref": "#/definitions/PayloadUser"
+        },
         "verified": {
           "type": "boolean",
           "x-go-name": "Verified"