Refactor generation of iCalendar files

[jennifer-richards]

Description

The datatracker generates iCalendar (RFC5545) compatible files in several places. These are generated via Django template interpolation. These templates involve quite a bit of conditional logic and wind up being fragile. I suggest investigating whether it's practical to offload generation of the iCalendar file to a library.

https://pypi.org/project/icalendar/ seems to be an obvious package to consider.

Code of Conduct

• [X] I agree to follow the IETF's Code of Conduct

[rjsparks]

@alexamirante reports that the current handcruft is breaking Thunderbird's calendar here:

DESCRIPTION:
 Note: Moving the meeting to review I-D draft-ietf-mpls-mna-usecases based on the authors request.
\

\
\n
 \nAgenda:
  https://datatracker.ietf.org/meeting/interim-2023-mpls-17/materials/agenda-interim-2023-mpls-17-mpls-01-00\n

[jennifer-richards]

It was pointed out in #7113 that the current mechanism for including timezone data includes daylight savings RDATE fields going back to the early 1900s. If possible, the timezone data should be pruned to relevant entries.