When a user is unable to merge an MR themselves, provide a list of users that can

This is what it looks like when a user doesn't have merge access for a repository:

I've been running into this lately. I don't know who can merge in a given project, so I don't know who'd be the best person to ping in a given project. It’d be great if you could click on “someone with write access to this repository” and open a dialog that’d show you all the users with merge access in the project.

As a "minimum viable product" I think it'd be fine to just have it list users, perhaps later we could make it so clicking on their name would assign the MR to them.

It looks like the text itself is in app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_not_allowed.js, if that helps anyone get started.