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/storage/PhabricatorFile.php

@@ -491,6 +491,12 @@ final class PhabricatorFile extends PhabricatorFileDAO
     return (string) $uri;
   }
 
+  public function getProfileThumbURI() {
+    $path = '/file/xform/thumb-profile/'.$this->getPHID().'/'
+      .$this->getSecretKey().'/';
+    return PhabricatorEnv::getCDNURI($path);
+  }
+
   public function getThumb60x45URI() {
     $path = '/file/xform/thumb-60x45/'.$this->getPHID().'/'
       .$this->getSecretKey().'/';