Avoid exception setting project profile image when GD not installed

Summary:
When trying to set a custom project profile image while the PHP GD extension is not installed, use the same logic which already exists in PhabricatorFilesComposeAvatarBuiltinFile.php to set the default project image. This stills display an unhelpful error message This server only supports these image formats: . but avoids an exception trying to call GD's imagecreatefromstring().

EXCEPTION: (Error) Call to undefined function imagecreatefromstring() at [<phorge>/src/applications/files/builtin/PhabricatorFilesComposeIconBuiltinFile.php:131]

Closes T15326

Test Plan:

1. Remove the php-gd (and potentially gd) packages on your system; restart httpd
2. Go to http://phorge.localhost/project/manage/1/
3. Select Edit Picture in the sidebar on the right to go to the Edit Project Picture at http://phorge.localhost/project/picture/1/
4. Set a custom icon and color and click the Save Image button
5. Get This server only supports these image formats: . but no exception anymore

Reviewers: O1 Blessed Committers, avivey, valerio.bozzolan

Reviewed By: O1 Blessed Committers, avivey, valerio.bozzolan

Subscribers: avivey, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15326

Differential Revision: https://we.phorge.it/D25515