Make markdown tables fill the message width (#2757)
`.prose table` was `display: block` so it could scroll horizontally. A block-level
table wraps its rows in an anonymous table box that shrinks to fit its content,
so prose's `width: 100%` never reached the columns, and the 16rem cell cap left
most tables far narrower than the message.
Tables are now wrapped in a `.table-scroll` div by the marked renderer. The
wrapper scrolls; the table keeps `display: table` and stretches to full width.
Table margins move to the wrapper so spacing around tables is unchanged, and
cells get `overflow-wrap: break-word` so long hashes and URLs wrap in place.
