Thanks to the latest version of go-webdav, this is now a thing. A lot of
operations (like creating a calendar) are not yet supported. But the
basics work fine. Note that multi-calendar means that different users
can each have their own calenders. Resource sharing is not yet
implemented either.
Includes the adding of a lot of debug logs, as issues are otherwise
pretty hard to figure out. The logging still needs to be made more
consistent, and probably cleaned up a bit in some places.
The filesystem storage backend now implements the required functions to
act as a basic CalDAV server. Some refactoring was done based on the
go-webdav development: introduce a UserPrincipalBackend, a new function
to serve the user principal URL, and more. See this PR for lots of
details: https://github.com/emersion/go-webdav/pull/62
Also adds a simple facility for debug output.
