Made resources concep tags into a directive

This doesn't change the layout of the tags to match the new design yet
See issue #128 for the designs/plan
Changed layout of resource concept tags to inline

See issue #128
Changed order of tags, requires before teaches

See issue #128
Changed text for resources lacking concepts

e.g., if a resource doesn't require any concepts
Merge pull request #138 from kennib/concept-tags

Concept tags