Support anniversaries in addition to birthdays

[rsjtdrjgfuzkfg]

Some users might want to also see anniversaries of their contacts in calendar form.

Note that "anniversary" is not well-defined and Thunderbird's implementation is currently not compliant with the vCard standard. It is thus not necessarily a good idea to actually implement this in the near future.

[from a user request received via mail]

[mdonz]

Thanks for your work on this TB add-on. I fully support the above request. As a former user of TB's "CardBook" add-on, I am missing lots of its functionality and extending your add-on to work with "anniversary" fields in TB calendar would help. Thanks

[dbrewood]

I'd love support for anniversary dates to be added too, it'd be awesome! I'd be looking to use it with the macOS address book.

[alibaubau]

I'd love too the display of the anniversary dates, in my case I appreciate the capacity to remember the "death" dates.
On my smartphone the LineageOS' Etar Calendar displays already anniversaries and birthdays.