name displayed in the repo providers dropdown list is GitLab.com even if GitLab hostname is overridden

[billux]

Bug description

Thanks to this PR, we can use other GitLab instances as repo provider by overriding the hostname attribute in the GitLabRepoProvider class.

Unfortunately, the name displayed in the BinderHub dropdown list of possible repository providers still mentions GitLab.com, which is very misleading for users.

I guess we just have to make the display_name attribute configurable too, but I'm unsure how I should use traitlets object in the Jinja template then.
It seems to be a really easy fix and would be happy to send you a PR to fix that.

[dlamoris]

a similar thing happens if github repo provider host is overridden - if the user pastes in the url instead of just user/repo, it will fail to launch since it doesn't remove the host first (on mybinder.org it'll remove github.com)
The only way right now may be to override the template to take out the instruction that pasting url is ok, but would be nice if the js can account for any url host configs.