calendar: Various improvements to the calendar API

[kewisch]

Various improvements to the calendar API

Fixes #56 among others

[kewisch]

I'm going to go ahead and pile on commits here until you have time to review. If it becomes too big please do review each commit separately or tell me to stop.

[kewisch]

Commit 2958e9a replaces extensionCapabilities with overrideCapabilities. We only want to save values that have been overridden, otherwise if you have e.g. attachments: false in your manifest for 1.0 and attachments: true in 1.1 then it will still use attachments: false.

This will potentially break people who used the experiment and rely on the previous behavior, not sure if we want to indicate that in some way.