[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)

options/locale/locale_en-US.ini

@@ -1244,6 +1244,7 @@ editor.new_branch_name_desc = New branch name…
 editor.cancel = Cancel
 editor.filename_cannot_be_empty = The filename cannot be empty.
 editor.filename_is_invalid = The filename is invalid: "%s".
+editor.invalid_commit_mail = Invalid mail for creating a commit.
 editor.branch_does_not_exist = Branch "%s" does not exist in this repository.
 editor.branch_already_exists = Branch "%s" already exists in this repository.
 editor.directory_is_a_file = Directory name "%s" is already used as a filename in this repository.