Show account cards for external accounts on "linked accounts" screen and "link new account"

Summary: Ref T1536. These can probably use some design tweaking and there's a bit of a bug with profile images for some providers, but generally seems to be in the right ballpark.

Test Plan:
{F46604}

{F46605}

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6210

src/applications/files/controller/PhabricatorFileTransformController.php

@@ -49,6 +49,9 @@ final class PhabricatorFileTransformController
     $unguarded = AphrontWriteGuard::beginScopedUnguardedWrites();
 
     switch ($this->transform) {
+      case 'thumb-profile':
+        $xformed_file = $this->executeThumbTransform($file, 50, 50);
+        break;
       case 'thumb-280x210':
         $xformed_file = $this->executeThumbTransform($file, 280, 210);
         break;