watch: add 'watch' support

watch: add 'watch' support

This patch provides front-end support for 'watch' feature:
watchadd.cgi: creates new project watchdogs for a user
watchedit.cgi: manage watchdogs of projects and users (view/edit/delete)
watchdel.cgi: deletes watchdogs for projects or users

In addition, two scripts provide the means to 'contact' watchers:
watchspam.cgi: contact a set of watchers for a project
watchabuse.cgi: report abuse of the contact system (auto-blacklister)

The README was updated to describe the storage format and locations of
the watchdog files, as implemented by the Git::RepoCGI::Watch module.
The config.pl module has also been updated to provide the default
watchdog path setting when (re)run with '-u'.