Complete auth provider styling / branding

[nwaldispuehl]

The last, stale branch of the upstream project—login-page-style-update—contains:

• refactoring of authconfig to authproviders
• additional name property for auth providers (kinda already done with Adds the auth methods 'name' field to the login buttons, if it differs from the auth type. #382 )
• branding information for auth providers
• branded login page

and would be an interesting addition to the project.

[GoliathLabs]

@T0biii As you are already working on quite a few UI changes, would you mind picking this one up?

[T0biii]

@GoliathLabs I had already looked at this once,
but I can't really say if I can do that since I'm not that experienced

Edit:
now i know what my Problem with this PR was. The mentioned branch has only go changes:
Place1/wg-access-server@master...login-page-style-update