Allow Atom event_filters as GET parameters

We use my dashboard Atom RSS feed to have our team notified of new pushes (since the Slack Integration does only work for issues for us (https://gitlab.com/gitlab-com/support-forum/issues/195 )), but unfortunately, we are unable to filter the messages getting into the feed, thus we are receiving issue events twice, which is confusing.

There is a feature (event_filters) that allows filtering, but requires a cookie to be set. For most RSS readers (including the Slack integrated one), cookies cannot be set.

I would suggest adding the option to add event_filters to the query string, and only parsing the cookie, if it was not set using the query string.

This might be a candidate for an easy fix and open for pull requests