refactored conferences controller, dividing responsabilities using an…

[carlosdanielpohlod]

… service and an query

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).

This PR refactors the show method into conferences_controller, moving some logic to a service and a query to separate better the responsibilities and improve the controller code legibility.

This commit is an refactor of #3037

Changes proposed in this pull request:

[carlosdanielpohlod]

@hennevogel