Improves accessibility with titles reflecting card headers (#656).

[bwatson78]

• app/helpers/*: refactors away some of the logic in the partials.
• _about_this_collection_block.html.erb and _metadata_block.html.erb: adds title element into wrapping div and dl.
• _about_this_item_block.html.erb and _find_this_item_block.html.erb: ditto above and refactors logic into helper.
• app/views/catalog/_access_and_copyright_block.html.erb: ditto two above, but also adds title to copyright disclaimer.
• app/views/catalog/_is_part_of_block.html.erb: ditto three above and inserts recommended dts that describe the dd below. (see https://www.w3.org/TR/WCAG20-TECHS/H40.html)
• _show_tools.html.erb and _this_item_contains_block.html.erb: uses localization for the titles.
• app/views/catalog/_view_items_in_collection_block.html.erb: same as above and inserts a dt for descriptiveness.
• config/locales/blacklight.en.yml: creates new localizations.
• spec/*: checks coverage.

[bwatson78]