METADATA: Add Available for Download Date term to ETDs

[shanitajohn]

ACCEPTANCE CRITERIA

• [x] I view my ETD on the single item page I should see an Available for Download Date
• [x] The Available for Download Date should show up only on the single item view
• [ ] ~The Available for Download Date should be bold~
• [ ] ~Attribute name should be embargo_end~
• [x] The predicate should be generated dynamically, not stored
• [x] The data type is a date
• [x] I should only be able to have one Available for Download Date
• [x] The Available for Download Date should be derived from the embargo
• [x] I should not be able to see the Available for Download Date on the search results page
• [x] I should not be able to facet on the Available for Download Date
• [x] When I view the .ttl I should be able to find the Available for Download Date

SPREADSHEET https://docs.google.com/spreadsheets/d/1HZpXyVM8xaZZ9mE2uzsbUmWKbCd6NYJTESOrpQeMa18/edit#gid=0

[little9]

@mark-dce @shanitajohn 'The Available for Download Date should be bold', does that mean that the label or the value should be bold? The labels in that table are all bold already.

[little9]

@mark-dce @tom https://github.com/curationexperts/mahonia/pull/176 addresses all of this except the label being bold and the date showing up in the TTL. Hyrax already has an attribute embargo_release_date, which is everything we want, except the ttl view. I'm not sure how to add that to graph.