The grid table parsers for markdown and rst was combined into one single
parser, slightly changing parsing behavior of both parsers:
- The markdown parser now compactifies block content cell-wise: pure
text blocks in cells are now treated as paragraphs only if the cell
contains multiple paragraphs, and as plain blocks otherwise. Before,
this was true only for single-column tables.
- The rst parser now accepts newlines and multiple blocks in header
cells.
Closes: #3638
