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 unessary before block in concept_spec.rb

#55