Update oauth button logos for Twitter and Google

Both new buttons are 64x64 renders of the recommended assets
from each company.

Refs for those are:

 - https://about.twitter.com/press/twitter-brand-policy
 - https://developers.google.com/identity/branding-guidelines

Addresses issue #544

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 7.12.0 (unreleased)
+  - Update oauth button logos for Twitter and Google to recommended assets
   - Update browser gem to version 0.8.0 for IE11 support (Stan Hu)
   - Fix timeout when rendering file with thousands of lines.
   - Add "Remember me" checkbox to LDAP signin form.