Alternative, short route for merge requests
### Description
Compared to routes like `/issues` or `/snippets`, `/merge_requests` is an outlier in terms of length and ease of typing.
### Proposal
Introduce alias at `/mr` which, importantly, isn't a mere redirect. This reduces the length and, by avoiding `_requests`, reduces likelihood of typos, too. Suggestions for alternatives that are not longer than, say, `/merges` welcome.
##### Before
https://server/org/proj/merge_requests
https://server/org/proj/merge_requests/123
##### After
https://server/org/proj/mr
https://server/org/proj/mr/123
### Why
Not everything involving a GitLab instance happens inside the web app, so it's important to have ergonomic and easy to remember/type urls, too.
issue