Use API endpoint to determine self hosted gitlab URL

Currently our code checks against gitlab.com to determine gitlab links, considering how we already have the self hosted gitlab instance API URL, we should be able to use that URL to determine gitlab links as well. We should make this adjustment in the next iteration

mentioned in issue #48

We'd really appreciate this, as we're trying to use the GitLab Trello power-up with our self-hosted GitLab Enterprise instance but it just looks like this:

For now we've forked and implemented a hacky fix (modifying isUrlGitLab in https://gitlab.com/gitlab-org/trello-power-up/blob/master/public/scripts/utils.js to reference our own hosted instance URL instead of gitlab.com) but we're not really sure how to implement a proper way of handling this so can't submit a MR.