refactored ConferencesController#calendar: separated calendar event c…

[carlosdanielpohlod]

…reation into an specific service

Checklist

• [ x ] I have read the Contribution & Best practices Guide.
• [ x ] My branch is up-to-date with the upstream master branch.
• [x ] The tests pass locally with my changes.
• [ x] I have added tests that prove my fix is effective or that my feature works(if appropriate).
• I have added necessary documentation (if appropriate).

Refactored conferences_controller#calendar: separated calendar event creation in a specific service. Also added a test to GET#calendar route.

Changes proposed in this pull request:

[carlosdanielpohlod]

@hennevogel (refactor of #3037 part I)

[hennevogel]

Done differently in #2742 now