hgweb: add websub template filter
The purpose of this new filter is to make it possible to partially replace the
functionality of the interhg extension. The idea is to be able to define regular
expression based substitutions on a new "websub" config section. hgweb will then
be able to apply these substitutions wherever the "websub" filter is used on a
template.
This first revision just adds the code necessary to load the websub expressions
and adds the websub filter, but it does not add any calls to the websub filter
itself on any of the templates. That will be done on the following revisions.
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.