Release Planning/0.9.9.8

Intentions
It is hoped that 0.9.9.8 will be mostly bugfixes with very little additional functionality, moving towards a 1.0 release. If sufficiently stable after release then we may skip 0.9.9.9 and release 1.0, or may deem 0.9.9.9 desirable for further testing.

Must Fix Bugs

 * Last fixes for numbers as usernames (fixed in Git).

Must Have Functionality

 * CalDAV Scheduling extensions: handle DELETE by ORGANIZER

Desirable Functionality

 * Updated translations
 * CalDAV Scheduling extensions: handle DELETE by ATTENDEE
 * Add support for DAV 'COPY'