[Mentions] Scroll so that post is directly above composer when clicking "Reply"

When you click "Reply" on a post that is towards the bottom of the screen, the post composer slides up over the top of it so you have to manually scroll down to see it again. We should do this automatically.

Start by reproducing the Mentions post reply flow with a post near the bottom of the viewport, then locate the reply action and composer positioning logic. Done means clicking Reply automatically scrolls the replied-to post directly above the composer without requiring manual scrolling.

Written by the indexing model from the issue text.