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
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![app/assets/images/authbuttons/google_64.png](https://lab.nexedi.cn/kirr/gitlab-ce/-/raw/190c9e9f39c6d14c2a47083fcd5d64717919d9eb/app/assets/images/authbuttons/google_64.png)