Each table row in a Markdown attachment is now its own row in the review UI, and so its own small table. Each one sized its columns to its own content, so columns didn't line up. This change makes it so all rows of a table now get the same fixed column widths, in `ch` units, based on the longest text in each column. Widths are capped at 40 characters, so long cells wrap. Cells get padding, and headers use the same font size as other cells so the widths fit. Testing Done: - Ran unit tests. - Viewed a Markdown attachment with a table, with and without a diff, and checked the columns lined up.