Update oauth button logos for Twitter and Google

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

Refs for those recommendations are:

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

Closes issue #544

[ci skip]

Sign in UI before:

![sign-in-buttons-before](https://gitlab.com/fmborghino/gitlab-ce/uploads/ed42a056b23eb6db3907cb604a865ce5/sign-in-buttons-before.png)

Sign in UI after:

![sign-in-buttons-after](https://gitlab.com/fmborghino/gitlab-ce/uploads/c7fb1d996fa744274298e195799bc033/sign-in-buttons-after.png)

See merge request !798