Ability to subscribe to issues as ICS calendar

[nodiscc]

• Gitea version (or commit ref): any
• Can you reproduce the bug at https://try.gitea.io:
  • [x] Yes

Description

Users should be able to subscribe to their assigned issues (or any other issue search) as an ICS calendar file. This allows integrating gitea issues/TODOs in one's own calendar/task management system (eg. Thunderbird or any calendar/task manager that supports ICS subscriptions)

For example, Gitlab has this feature:

• Enter any search query from the issues dashboard, eg https://gitlab.com/dashboard/issues?assignee_username=nodiscc
• An ICS link appears in the top right corner of search results

• The link points to an ICS file (protected by a unique token) https://gitlab.com/dashboard/issues.ics?assignee_username=nodiscc&due_date=next_month_and_previous_two_weeks&feed_token=XXXXXXXXX&sort=closest_future_date&state=opened. Example contents:

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//GitLab//NONSGML GitLab//EN
CALSCALE:GREGORIAN
X-WR-CALNAME:GitLab Issues
BEGIN:VEVENT
DTSTAMP:20201120T184651Z
UID:AAAA-BBBB-CCCC-DDD-2b12cfece978
DTSTART;VALUE=DATE:20201121
DESCRIPTION:Find out more at https://gitlab.com/nodiscc/awesome-linuxaudio/
 -/issues/7
SUMMARY:Move CI/syntax checks to gitlab CI (in nodiscc/awesome-linuxaudio)
TRANSP:TRANSPARENT
URL:https://gitlab.com/nodiscc/awesome-linuxaudio/-/issues/7
END:VEVENT
END:VCALENDAR

• After subscribing to this ICS file from a calendar management program, issues are displayed according to their due date property on the calendar view:

[nodiscc]

Note that Gitlab only implements the VEVENT ICS object type (calendar event - DTSTART value is required). VTODO is another widely used object type (todo list entry with or without a due date), but is not implemented. In my opinion this is fine and there is no need to implement the whole ICS specification :upside_down_face:

[Zocker1999NET]

I would like having support for VTODO's because this would make it possible to represent issues without due dates and to display them in task apps instead of (just) your calendar. Maybe both could be implemented and the user might choose which link the user wants. But this can happen in a second step, implementing any support for ICS export would be a good start.

Has Gitea already an infrastructure for creating & managing such tokens?