Nice fixes/enhancements from Hoffi for JEM 1.9.5

Things which should be part of 1.9.5:
- delete obsolete files on update (issue #275)
- alter catids and myattending url in menu table and catids in module table when crossing 1.9.4 -> 1.9.5 border
- correct mailer plugin (issue #522)
- change 'el*' to 'jem*' in search plugin to allow searching in EL and JEM at same time
- ACL view access - support Joomla access levels instead of single gid
- fix archive button creation (in case old events are not configured to move into archive)