calendar readonly cannot be unset by user

[opto]

84f9081, still relevant?

experiment creates an readonly calendar, and the UI cannot unset readonly in calendar properties (is greyed out)

[opto]

this seems to be relted to the calendar being immutable, when I look at the code, so this issue may be obsolete

[kewisch]

It seems to me the default if not specified should be that calendars are mutable. There are two options:

1. Rename the property from mutable to immutable. If immutable is not specified or explicitly false, everything is normal
2. Fix the check from !mutable to mutable === false so that only explicitly false makes the calendar immutable.