users: move account deletion endpoint to /users/:id/deactivate.

Move the account deletion endpoint from /maintenance/users/deletion to either:

* https://danbooru.donmai.us/users/deactivate
* https://danbooru.donmai.us/users/:id/deactivate

This incidentally allows the Owner-level user to deactivate accounts belonging to other users. This
is meant for things like deactivating inactive accounts with invalid or abusive names. This is
limited to accounts below Gold level for security.

app/views/users/edit.html.erb

@@ -53,7 +53,7 @@
         <div class="input">
           <label>Account</label>
           <p>
-            <%= link_to "Deactivate account", maintenance_user_deletion_path, id: "delete-account" %>
+            <%= link_to "Deactivate account", deactivate_user_path(@user), id: "delete-account" %>
             (<em>requires confirmation</em>)
           </p>
         </div>