Support GitHub Enterprise for the Pull Requests feature (#5596)

The feature to show PR icons in the branches list only worked for
github.com, but not for GitHub Enterprise remotes. This PR makes that
work, if you configure a services entry for it as explained
[here](https://github.com/jesseduffield/lazygit/blob/master/docs/Config.md#custom-pull-request-urls).

Prior art: #5559.