Document upgrade path to achieve common model gem

[mark-dce]

• [x] Document current state of MIRA/TDIL (tufts_models), DL/Corpora(hydrafication-tufts)
• [x] Document known steps necessary to consolidate tufts_models & hydrafication-tufts
  • [x] includes update DL/Corpora/hydrafacation to current blacklight & hydra
• [x] Document steps to migrate repository data for explicit namespaces
• [x] Document steps to update model gem to support explicit namespaces (i.e. see branch in tufts_models)

NOTE: This tickets is about writing a 1-2 page summary of all the information we've collected so far and handing it off in a pretty document.