Reduce the amount of OAuth1/OAuth2 code duplication for rendering login buttons

Summary: Ref T3687. These buttons don't work quite the same way, but are similar enough that the code seems worth consolidating.

Test Plan: Viewed and clicked both OAuth1 (Twitter, JIRA) and OAuth2 (Facebook) login buttons. Got logins.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3687

Differential Revision: https://secure.phabricator.com/D6874