Allow tables if HTML::Gumbo is installed

HTML::Gumbo deals with ensuring that content cannot "escape" from the
context that RT frames it in, by (for example) not allowing </td></tr>
if the content has not opened its own table.  HTML::Gumbo has an
HTML::Parser-like interface, but it is not quite close enough to serve
as a drop-in replacement -- and the structure of HTML::Scrubber would
not make such a substitution easy.

As such, pre-parse the HTML content using Gumbo, if available, as a
pre-parsing step before HTML::Scrubber.  This enables <table> tags and
their ilk to be enabled without posing a security risk.
Allow additional CSS rules for borders and padding
Document that RT scrubs HTML, and note that HTML::Gumbo loosens those slightly
Merge branch '4.2/html-gumbo-balancing' into 4.2-trunk