Add maintenance script to fix missing menu links
Don't build system link when editing it

Some links such as eticket download cannot be built without certain
conditions being met (e.g. the current user being registered in the
conference).
Also, the link was just built but not shown anywhere...
Cache PluginOption.getRooms during request
Remove some unused cache utils
Make email body show up properly in log entries
Add seems_html helper to detect MIME in old emails