Added GitHubApiUrl and GitHubDomain to settings and template
Put values from settings into handler/auth.go
Use custom github domain for webhook
Update all github clients in code
Added settings to github_add page (fix redirect)
Fixed identation
Make IsGit check less strict
Fixed another github client instance in code
Migration file based on #83

I choosed Rev3, because Rev2 will be about "open invitations" feature.
Merge pull request #75 from floatdrop/github-enterprise-support

GitHub Enterprise Support