Fixed-width Issue and MR pages don't interact well with "Layout width" Fixed/Fluid user preference

When the "Layout width" option on https://gitlab.com/profile/preferences is set to "Fluid", the issue and MR description doesn't change to fixed, but the discussion/diffs do:

Issue description is fixed, but discussion is fluid

MR description is fixed, but diffs are fluid

I think that description fixed-ness should follow the user's "Layout width" setting, like the rest of the app.