refactor(gitlab_project): fetch project.webUrl to fix openers

This fixes an unreleased issue where the extension couldn't open the GitLab website.

In !168 (merged) we started fetching GitLab projects through GraphQL but we forgot to fetch project.web_url. And the compilation didn't warn us because the code that depended on web_url was all in JavaScript. For example, creating a new MR:

openUrl(`${project.web_url}/merge_requests/new?merge_request%5Bsource_branch%5D=${branchName}`);

Resulted in

The fix is to fetch the webUrl attribute

I chose the MR title to be refactor: (as opposed to fix:) because it's fixing an unreleased issue and I would prefer not to have it shown in the CHANGELOG (refactorings don't). This will prevent users from thinking that the release contains a bugfix when actually there wasn't a released bug.

Related to #143 (closed)