OR-697 Update the documentation
OR-697 Add a backend core module in order to dispatch event

* Only event `created` is implemented, other type will return an error.
* Add unit and storage tests.
* Update module-manager unit test.
OR-697 Create a POST route to handle calendar event creation

* Add the route `/api/calendars/:id/events`.
* Add the associated controller
* Add midway tests
Merge pull request #543 in OR/rse from ~RPIGNOLET/rse:OR-697 to master

* commit 'b41f34d4b4e4b2d01300fdd87f9e34c6079771c3':
  OR-697 Create a POST route to handle calendar event creation
  OR-697 Add a backend core module in order to dispatch event
  OR-697 Update the documentation