Oversized topics should be scrollable

[matrixbot]

Created by @ matthew:matrix.org.

[JasonLocklin]

Or expand/hide-able. Sometimes you want to see the whole thing, other times, screen-space is limited (such as on small devices).

[richvdh]

dup #1725

[richvdh]

wait, not a dup. #1725 has been replaced with a different problem.

[ara4n]

there's a PR from will for this, but came with provisos: #2287