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>

templates/pkgbase/comaintainers.html

@@ -0,0 +1,40 @@
+{% extends "partials/layout.html" %}
+
+{% block pageContent %}
+    {% if errors %}
+        <ul class="errorlist">
+            {% for error in errors %}
+                <li>{{ error | tr }}</li>
+            {% endfor %}
+        </ul>
+    {% endif %}
+
+    <div class="box">
+        <h2>{{ "Manage Co-maintainers" | tr }}:</h2>
+        <p>
+            {{
+                "Use this form to add co-maintainers for %s%s%s "
+                "(one user name per line):"
+                | tr | format("<strong>", pkgbase.Name, "</strong>")
+                | safe
+            }}
+        </p>
+
+        <form action="/pkgbase/{{ pkgbase.Name }}/comaintainers" method="post">
+            <fieldset>
+                <p>
+                    <label for="id_users">{{ "Users" | tr }}:</label>
+                    <textarea id="id_users" name="users"
+                              rows="5" cols="50">{{ "\n".join(comaintainers) }}</textarea>
+                </p>
+
+                <p>
+                    <button class="button" type="submit">
+                        {{ "Save" | tr }}
+                    </button>
+                </p>
+            </fieldset>
+        </form>
+
+    </div>
+{% endblock %}