Default margins of content elements in IRC layout

After the rejection of [this PR](https://github.com/matrix-org/matrix-react-sdk/pull/9134) I was encouraged to start a discussion about the problem in an issue here.

So there are default margins set for these content elements: `blockquote`, `dl`, `ol`, `p`, `pre`, `table`, `ul`. These default margins are removed with the IRC layout, but as far as I can see *only for the bottom margin of* `blockquote`. Here are the recycled screenshots from the PR:

overridden/removed bottom margin:


default margin:


As I stated in the PR, it looks pretty ugly imho to have a blockquote have space above (bottom margin of previous element) but not a space below (overridden/removed default margin).

I was encouraged to start a discussion about whether that removal of default margins should be placed on *all* these elements in order to have the IRC layout be as compact as possible, or otherwise do not remove it from the blockquote bottom.