Fixed some margins and widths on the issue page

What does this MR do and why?

I noticed that some margins were not right on the issue page and caused some misalignments.

This MR fixes that 🙂

While fixing the misalignment I also noticed that the mobile version of the page had some problems so I fixed them in this MR too, to avoid conflicts with two separate MRs 🙂

I also took the chance to use the SCSS mixins provided by the gitlab-ui instead of hard-coded values 🙂

Screenshots or screen recordings

These are the screenshots of the original misalignment I noticed

	Original	Modified
Guest user
Non guest user (same layout as before)

These are the screenshots of the fixes I did for the mobile

	Original	Modified
Guest user
Non guest user

How to set up and validate locally

To validate the alignment changes you can just impersonate a guest user and check an issue.

To validate the mobile changes you can check any issue both impersonating a guest and impersonating a user with non-guest access to the repo.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.