Add apply-patch, basic revert and cherry-pick functionality (#17902)

This code adds a simple endpoint to apply patches to repositories and
branches on gitea. This is then used along with the conflicting checking
code in #18004 to provide a basic implementation of cherry-pick revert.

Now because the buttons necessary for cherry-pick and revert have 
required us to create a dropdown next to the Browse Source button
I've also implemented Create Branch and Create Tag operations.

Fix #3880 
Fix #17986 

Signed-off-by: Andrew Thornton <art27@cantab.net>

web_src/js/features/repo-legacy.js

@@ -436,7 +436,7 @@ export function initRepository() {
   });
 
   // File list and commits
-  if ($('.repository.file.list').length > 0 ||
+  if ($('.repository.file.list').length > 0 || $('.branch-dropdown').length > 0 ||
     $('.repository.commits').length > 0 || $('.repository.release').length > 0) {
     initRepoBranchTagDropdown('.choose.reference .dropdown');
   }