Handle caldav color for calendars

[bertrandda]

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Description of change

To synchronize color of the calendar inside CalDAV service.
julienL post: https://community.gladysassistant.com/t/calendrier-module-caldav/4598/58

Tested with Nexctloud, iCloud, Google Calendar

[codecov]

Codecov Report

Merging #899 (ca62279) into master (e467c86) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #899   +/-   ##
=======================================
  Coverage   94.65%   94.65%           
=======================================
  Files         494      494           
  Lines        6552     6552           
=======================================
  Hits         6202     6202           
  Misses        350      350           

Impacted Files	Coverage Δ
server/models/calendar.js	100.00% <ø> (ø)
server/seeders/20190403081927-calendar.js	100.00% <ø> (ø)
...services/caldav/lib/calendar/calendar.formaters.js	90.62% <ø> (ø)
.../services/caldav/lib/calendar/calendar.requests.js	100.00% <ø> (ø)

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[Pierre-Gilles]

When starting the frontend locally with a DB with existing event, I got the following crash in the UI:

Uncaught (in promise) TypeError: Cannot read property 'substring' of null
    at isBright (color.js?d8c9:2)
    at Map._this.eventPropGetter (index.js?3b01:39)
    at Object.eventProp (react-big-calendar.esm.js?496b:4411)
    at EventCell.render (react-big-calendar.esm.js?496b:268)
    at N (preact.js?10a9:1)
    at eval (preact.js?10a9:1)
    at k (preact.js?10a9:1)
    at g (preact.js?10a9:1)
    at T (preact.js?10a9:1)
    at N (preact.js?10a9:1)

[Pierre-Gilles]

Thanks for your PR, it works fine!