[FR] Add CalDAV support.

[RokeJulianLockhart]

Please describe the problem / need you are trying to solve.
I want to be able to access my tasks via desktop clients.

Describe the feature or the improvement you'd like and what are you trying to achieve.
Add CalDAV support to allow this. Currently, the tasks are locked into the Taiga server and unsynchronizable (and consequently unmanageable) elsewhere, and/or offline (outside an intranet).

Describe alternatives you've considered
A custom API, although this isn't a solution. It's a bodge.

Additional context
Every other task manager provides this.

Relevant:

0. https://gitlab.com/gitlab-org/gitlab/-/issues/14390
1. [Feature Request] Task support (CalDAV?) raindropio/app#82 (comment)
2. [Feature Request] Expose lists via CalDAV. wishlephant/wishlephant-website#1 (comment)
3. [Enhancement] Expose user lists via CalDAV. bookwyrm-social/bookwyrm#2952 (comment)
4. Expose user lists via CalDAV. recloudstream/cloudstream#550 (comment)
5. Expose user lists via CalDAV. LagradOst/QuickNovel#184 (comment)
6. Expose wishlist entries as CalDAV tasks. ValveSoftware/steam-for-linux#10019 (comment)