various: add read-only mode support

various: add read-only mode support

By creating a file named "readonly" located at
$Girocco::Config::chroot/etc/readonly, external pushes and project
modifications via the web UI will be prohibited.

Modifications via local tools such as projtool or usertool are NOT
prohibited by this read-only mode, nor does this mode stop jobd or
taskd from running.

The text on the home page of the UI may be automatically altered
as well provided an alternate version of the index text file has
been made available via the gitweb_config.perl $home_text_ro setting.

If the $Girocco::Config::chroot/etc/readonly file is not empty, its
contents will also be included in the messages denying any changes.
Except for the home page, which will only show a change if $home_text_ro
points at an alternate file.