[GITEA] Allow user to select email for file operations in Web UI

- Add a dropdown to the web interface for changing files to select which
Email should be used for the commit. It only shows (and verifies) that a
activated mail can be used, while this isn't necessary, it's better to
have this already in place.
- Added integration testing.
- Resolves https://codeberg.org/forgejo/forgejo/issues/281

(cherry picked from commit 564e701f40)
(cherry picked from commit de8f2e03cc)
(cherry picked from commit 0182cff12e)
(cherry picked from commit 9c74254d46)

services/forms/repo_form.go

@@ -767,6 +767,7 @@ type EditRepoFileForm struct {
 	CommitChoice  string `binding:"Required;MaxSize(50)"`
 	NewBranchName string `binding:"GitRefName;MaxSize(100)"`
 	LastCommit    string
+	CommitMailID  int64 `binding:"Required"`
 	Signoff       bool
 }