Improve text wrapping on long table data

Default to word-wrapping on header rows, anywhere-wrapping on content (to handle long user input).

The boardtable nth-child() rules for PPH, PPD, Users and Posts are no longer needed, and the nth-child() rule for Description was preventing wrapping in the global management boards table when there was an excessive unbroken description.

The td pre rule was never in use (only applies to a hardcoded message on the home page and the FAQ, neither at risk of containing long content)

Tested on both wide and narrow/mobile window sizes.