Require SabreDAV 2.1+, update basic auth handlers
Properties plugin: update to SabreDAV 2.x event handlers

The afterGetProperties event that we relied on in SabreDAV 1.8+ no
longer exists; use the new propFind and propPatch events to add our
custom properties.
BrowserPlugin: update to inherit from SabreDAV 2.1
Directory: remove unneeded variable declarations
File: remove unneeded variable declarations