Clean up image loading for ExternalAccounts

Summary: Ref T1536. This gets the single queries out of the View and builds a propery Query class for ExternalAccount.

Test Plan: Linked/unlinked accounts, logged out, logged in.

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6212