refs #7688
Adds logic in theme settings api to either serve an uploaded favicon and give it the type `upload` or use the default settings `default`, which will serve the favicon from our shared directory.
TODOs for #7688:
- [X] Figure out, which favicon should be used (uploaded or default) -> this PR
- [ ] Serve and redirect the favicon for any browser requests, incl. redirects
- [ ] Upload favicon via `general/settings` and implement basic admin validations -> [WIP] TryGhost/Ghost-Admin#397
- [ ] Built server side validations
