Show self-hosted instance's logos in the account switcher (instead of the globe icon)

In the quick-switcher as well as it's fullscreen view, there are currently gray globe icons for selfhosted instances

The icons could be replaced with the instance's own logo

Related to https://feedback.fluxer.com/p/3653/c/8798#c-8798, which requests the instance name to be shown on the quick switcher as well. Hampus already talked about this here: https://feedback.fluxer.com/p/3625/c/8351 I'm just opening the FR because nobody else has, just to write things down properly :)