"Create Merge Request" feature lacks feedback

On issue pages without an associated merge request, we now have the option to simultaneously create a new branch and create a merge request to go with that branch.

I recently tried out this feature and here's how it went:

1. clicked the button
2. the button was disabled, but nothing appeared to happen
3. getting impatient, I refreshed the page
4. the page listed that a new branch had been created associated with the issue, but no MR. I assumed the operation to create the merge request failed.
5. I clicked on the branch name, and then clicked "create merge request" from the branch page, then I filled out the new merge request form and attempted to submit
6. I got an error stating that another MR with this name already existed. It was apparently the one I attempted to create the first time around by clicking the button on the issue page!

Given that the operation of creating a branch and then creating a merge request is not instantaneous, we ought to have some sort of feedback that this operation is in progress. Here's what I propose:

• Instead of just disabling the button after clicking, we should put a loading spinner in the button.
• If possible, we should also maintain some state for this process within the issuable object so subsequent page refreshes still show this as in-progress.