mergeInProgress JS can redirect user to an invalid URL

Something to keep an eye out for, not 100% sure on reproduction yet:

1. I went to a MR from an email notification, it pointed to a note: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2153#note_2990776
2. I checked "Remove source branch"
3. Hit the drop-down and then "Merge Immediately"
4. The browser URL changed to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/note_2990776?delete_source=true (note lack of MR ID and hash for the note anchor)
5. The "Merge in progress" message never disappeared, likely due to the now-invalid URL.

cc @rdavila @DouweM

Related MR: !1897 (merged)

Reassigned to @rdavila

I was able to reproduce the issue from the MR's detail page, working on a fix now.

mentioned in merge request !2169 (merged)

mentioned in commit 42da7cd3

Status changed to closed by commit 541da4d5

mentioned in commit 541da4d5