Calendar viewer plugin for Providence : display occurrences or objects contemporary dates on a calendar

Hello,

Here comes the Pull Request for the calendar viewer plugin.

Plugin to display contemporary dates for occurrences, loans or movements on a day/week/month calendar.
Allows the ability to display an additional phase before and after the event, to visualize preparation and cleanup phases for exhibition management.

To see what's impacted with fullcalendar upgrade : 
https://github.com/gautiermichelin/providence/commit/72381d31153fb11d6b2365a25fc1816e615095c8
https://github.com/gautiermichelin/providence/commit/c0e429d51ea9a64cf417b6110a595695dcad7cd9
https://github.com/gautiermichelin/providence/commit/75bae5e950f0c6b9578e6b3c487c8b9dc6d1b256

To see files corresponding to the new calendar viewer plugin : 
https://github.com/gautiermichelin/providence/commit/77edd11bdfc5838f8e2a3ef12e8ad0eced9468d3
https://github.com/gautiermichelin/providence/commit/ca809e4a2d74bdfcffa38487cf9b4e514faa975c
https://github.com/gautiermichelin/providence/commit/2f4fe86fc197bebf477c9a8df3ad53b1dddd03d7

Bye,

Gautier