Add a link to a list of known f-droid repos to the client

The obvious place for the link would be in the settings under 'My Apps'

Linking to either the wiki page or the discord thread seems like a decent idea. This doesn't explicitly endorse any known repos, and gives ample space to document whether or not the user should trust any of the repositories.

Currently the discord thread is more readable, but has the downside of being easy to post information in for anyone. The wiki-page needs some work in order to become more mobile-friendly, but is easier to keep under control by the f-droid team.