Automatically assign MR creator as assignee when creating MR through gitlab UI

Proposal

When a user creates a merge request directly in gitlab from a project view, automatically assign them as the assignee. Basically, do exactly what is described in #287636 (closed) except apply it when creating a merge request directly in the gitlab UI.

Benefit

For users that are using Jira (or other ticketing systems) that don't benefit from #287636 (closed), they can avoid having to assign themselves manually to every MR they create.