feat(FastAPI): add /pkgbase/{name}/comaintainers (get, post)

Changes from PHP:

- Form action now points to `/pkgbase/{name}/comaintainers`.
- When an error occurs, users are sent back to
  `/pkgbase/{name}/comaintainers` with an error at the top of the page.
  (PHP used to send people to /pkgbase/, which ended up at a blank
  search page).

Closes: https://gitlab.archlinux.org/archlinux/aurweb/-/issues/51

Signed-off-by: Kevin Morris <kevr@0cost.org>

templates/partials/packages/actions.html

@@ -117,9 +117,9 @@
 
                 {% endif %}
             </form>
-            {% if is_maintainer %}
+            {% if request.user.has_credential('CRED_PKGBASE_EDIT_COMAINTAINERS', approved=[pkgbase.Maintainer]) %}
                 <li>
-                    <a href="/pkgbase/{{ result.Name }}/comaintainers/">
+                    <a href="/pkgbase/{{ pkgbase.Name }}/comaintainers/">
                         {{ "Manage Co-Maintainers" | tr }}
                     </a>
                 </li>