Rework Oauth login buttons, swap github logo to monocolor (#24740)

Rework Oauth login buttons, swap github logo to monocolor (#24740)

Diff without whitespace:
https://github.com/go-gitea/gitea/pull/24740/files?diff=unified&w=1

- Use SVGs for GitHub and GitLab oauth providers
- Replace section wrapping with a divider
- Rework icon rendering, increase size from 32px to 40px

Before:

<img width="853" alt="Screenshot 2023-05-15 at 21 54 23"
src="https://github.com/go-gitea/gitea/assets/115237/6ab5cfb4-46ff-469a-bd1f-06780d4a6a0b">

After (more providers):

<img width="849" alt="Screenshot 2023-05-15 at 21 51 21"
src="https://github.com/go-gitea/gitea/assets/115237/fa84f92f-98e0-4aed-9357-5d62ddd98195">

<img width="856" alt="Screenshot 2023-05-15 at 21 56 45"
src="https://github.com/go-gitea/gitea/assets/115237/d3edd7ed-dadd-4302-aca7-08f20adc220e">

Ref: https://codeberg.org/Codeberg/Community/issues/1023

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Co-authored-by: Giteabot <teabot@gitea.io>