events: allow rss feeds to show all events

RSS feeds can be easily parsed and hence can be used to populate program for
events with the proper tags. By adding the 'showall=true' to the query string,
it is possible to see all events corresponding to the requested criteria in
the RSS feed, whether past or future and more than 20.

Signed-off-by: Geneviève Bastien <gbastien@versatic.net>

See merge request !1