Add support for account switching

[wesharper]

Right now, if I want to switch between work and personal projects, it feels a bit awkward as I have to choose which gitbutler account to sign in with. Logging out and logging back in is too cumbersome, but it feels wrong to be using my work account on personal stuff and vice-versa, even if the git authentication is handled via credentials-manager.

Ideally, different accounts could store different user-level config (even different git configs) and I could switch them in the UI to change the "author" of the commit and throw errors/warnings if the logged in user cannot commit/push to the repo.