Branded appearance to CE

Closes #11489 

The difference with the EE version is only that there is no distinction between light and dark logos, though this wasn't used anyway. If this is fine, I'll create a MR on EE too.

TODO:
- [x] Copy docs
- [x] Make new screenshots
- [ ] Remove Custom Welcome message feature?

@rymai: I was unsure what labels to add to ping you, so I just ping you like this 😉 

/cc @DouweM

See merge request !2927