Add Webcals synchronization

[bertrandda]

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Description of change

Please provide a description of the change here. It's always best with screenshots, so don't hesitate to add some!

Add synchronization for Webcals associate to CalDAV account.
Also add description in calendar.event model and use it in scenes.

[codecov]

Codecov Report

Merging #1537 (f6f76d4) into master (d45a7aa) will increase coverage by 0.02%.
The diff coverage is 97.95%.

@@            Coverage Diff             @@
##           master    #1537      +/-   ##
==========================================
+ Coverage   96.57%   96.59%   +0.02%     
==========================================
  Files         618      628      +10     
  Lines        8992     9205     +213     
==========================================
+ Hits         8684     8892     +208     
- Misses        308      313       +5     

Impacted Files	Coverage Δ
server/lib/scene/scene.actions.js	97.88% <ø> (ø)
server/lib/scene/scene.checkCalendarTriggers.js	100.00% <ø> (ø)
server/models/calendar.js	88.88% <ø> (ø)
server/models/calendar_event.js	100.00% <ø> (ø)
server/seeders/20190403081927-calendar.js	100.00% <ø> (ø)
server/seeders/20190403081933-calendar-event.js	100.00% <ø> (ø)
...es/caldav/lib/calendar/calendar.syncUserWebcals.js	96.77% <96.77%> (ø)
server/lib/calendar/calendar.get.js	100.00% <100.00%> (ø)
server/services/caldav/api/caldav.controller.js	100.00% <100.00%> (ø)
server/services/caldav/index.js	95.34% <100.00%> (+0.90%)	⬆️
... and 17 more

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[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[Pierre-Gilles]

Looks fine to me, works on my side!

[relativeci]

Job #460: Bundle Size — 6.85MB (+0.01%).

c80a3b6 vs cfe8d89

Changed metrics (2/10)

Metric	Current	Baseline
Initial JS	2.89MB(+0.03%)	2.89MB
Cache Invalidation	41.99%	58.69%

Changed assets by type (1/7)

	Current	Baseline
JS	4.88MB (+0.02%)	4.88MB

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View Job #460 report on app.relative-ci.com