Validate if user is able to push

[NiklasBuchfink]

Context

Users can't push their changes if the inlang GitHub app is not installed and they can't install it themselves if they are not the repo's owner. They will also lose their translations as there is no export or caching yet.

Related:

• FINK-40
• FINK-41
• FINK-51
• https://discord.com/channels/897438559458430986/1247865778976329821/1247891530203791370

Proposal

Lix should provide an API that detects if Lix can push to the repo

This way Fink (and other apps) can prompt to update the permissions (user is the owner), contact the repo's owner (user is a collaborator) or fork the project (if not disabled LIX-57 ).