Reduce database round-trips in Concept view (Issue #55)

On the Concept#show view concept_descriptions were
being loaded individually causing N + 1 queries. Eager
loading is now used to reduce the total number of calls.

*Eager load concept descriptions in ```ConceptsController#show```
*Refactor ```concept#concept_descriptions``` to be in
descending order
*Remove  ```concept#history```
*Fix issue with failing concept_features test for markdown
*Remove unnecessary before block in  ```concept_spec.rb``

#55