Improve display of long metadata value lists

For metadata that can have multiple values (eg therapeutic_area for a
drug_safety_update document) place each value into a separate `dd`.
Merge pull request #262 from alphagov/improve-display-of-metadata

Improve display of long metadata value lists
un-aaib-ify; comment out some redundant features
Various changes to make progress on DSU import
Remove debugging puts statements
Remove date_of_occurrence code in DSU mapper. Causes issues with non-unique slugs.
Prune more AAIB-specific code
provide therapeutic area, obliterate Report from DSU classnames
provide summary import
add dentistry to DSU schema