[Feature] Github Enterprise support

[achernenko-schibsted]

Is your feature request related to a problem? Please describe.
I love the idea of an app that lets me track PRs but I can't use PR Focus because our company hosts code on a Github Enterprise instance, so when I try to add a repository the app complains about it not being a valid Github URL:

We could not parse a valid GitHub URL from the provided URL: [repo url]

Describe the solution you'd like
Ideally, I would expect the app to support multiple Github instances with separate tokens, but having a global setting for the Github instance URL would work as well.