Consider a sticky header for MR page for mobile / narrow viewports

Proposal

There should be a sticky header for the MR page on mobile. Originally this concern was raised in !193105 (merged) to address file diff navigation however this could also include access to the "your review" as well.

Common thing here seems to be how might we open drawers of information in the MR on mobile.

Context

The following discussion from !193105 (merged) should be addressed:

• @mle started a discussion: (+1 comment)

  @slashmanov question in mobile on the Changes tab what should be sticky?

  Currently what I see in the mobile view is something like this on initial load

  

  Would something like this make sense to be be sticky for the Changes tab?

  

  As for the icon to open the files to differentiate from the sidebar itself, what if we used the icon that the files are listed either tree or list

  Tree	List