Explicitly link merge reuqest to issue(s)

At the moment, a merge request is defined as related to one or more issues when the user mentions an issue on the MR. This mention could happen due to various factors, and something as simple as saying This was already discussed in #... would "connect" the issue to the MR.

This, however, basically prevents us from building logic that depends on a "real" link between an issue and one, or more, merge requests. One simple example would be knowing which MRs were needed to solve/fix/close an issue.

The currently existing logic would put together MRs that are really needed to MRs that might simply reference the issue for other purposes.

Proposal

Add a new widget to the issue page that would have a similar behavior to what the work items does, allowing users to manually choose the MRs an issue it linked to.

The widget could have the Linked merge requests title, to differentiate it from the Related merge requests already present (which would keep the existing logic of automatically showing the MRs that mention the issue).