People: add a permalink to list Administrators

Summary:
This change introduces a builting query to easily list
Administrators of your beautiful Phorge instance:

{F277912}

Note that any builtin query can be disabled anytime or
reordered, so this is not a problem if you already have
a similar saved query. But probably you have not.

The goal is not to just "have a link" (this was already
possible) but to have a predictable link to be used in
any instance-agnostic document, including but not
limited to Diviner or Phorge's user interface messages.

Closes T15238

Test Plan: Visit /people/query/admin/ and enjoy.

Reviewers: O1 Blessed Committers, Cigaryno, Matthew

Reviewed By: O1 Blessed Committers, Cigaryno, Matthew

Subscribers: speck, tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15238

Differential Revision: https://we.phorge.it/D25116

src/applications/people/query/PhabricatorPeopleSearchEngine.php

@@ -200,6 +200,7 @@ final class PhabricatorPeopleSearchEngine
   protected function getBuiltinQueryNames() {
     $names = array(
       'active' => pht('Active'),
+      'admin' => pht('Administrators'),
       'all' => pht('All'),
     );
 
@@ -221,6 +222,9 @@ final class PhabricatorPeopleSearchEngine
       case 'active':
         return $query
           ->setParameter('isDisabled', false);
+      case 'admin':
+        return $query
+          ->setParameter('isAdmin', true);
       case 'approval':
         return $query
           ->setParameter('needsApproval', true)