Improve navigation in large merge requests

Description

Navigating merge requests with large descriptions is pretty annoying since the navigation bar isn't visible until you scroll down a lot. It's especially maddening when working on Gitlab itself and you have several screenshots of the Gitlab UI inside the description 😱

Proposal

A simple solution would be to simply give the description its own tab. Here's a quick mockup:

Description tab:

Discussion tab:

You could also keep the description and discussion on the same page and make the tabs scroll between the two. Now that I think about it that would probably make more sense, and would also avoid diverging from the Issues look too much.

Links / references

I couldn't find any related issues.