Mobile UI mega issue #4702

Many features just unavailable from mobile view right now that I bothered to create issues for each one of them

• PR diff view is too small. I can only see 13 lines at once from my phone. Most of the diff options can be hidden under one dropdown(?) button here. I don't think comment form and actions deserve this much accessibility too.

  

• Many text inputs has too small font size. <16px forces Safari to zoom in iirc

• no way to see or edit issue assignees

• no way to mark notification as viewed without clicking it.

• reactions button invisible for PR comments

• wrong header icon color

• markdown editor should be sticky

• image lightbox doesn't support swipe and only works with buttons.

There are few more I don't remember, will add while using it.