[ENG-4378] elasticsearch8

[aaxelb]

• add elasticsearch8 to local dev environment
• add share.search.index_strategy to act as a slippery abstraction layer between search-engine backend and planned friendly search api
• configure two index strategies (and make it easy to add more in the future):
  • sharev2_elastic5: the existing/legacy SHAREv2 search index as exists on elasticsearch5 and exposed via /api/v2/search/creativeworks/_search
  • sharev2_elastic8: a mirror/replacement for sharev2_elastic5 with all the same _source docs (but possible incompatibilities for the existing pass-thru api)
• add a happy-path index-backfill workflow to the admin interface at /admin/search-indexes
  • when changing index-strategy settings/mappings/whatever, the "happy path" is to create, backfill, verify a new copy of the index; then switch which is used for searching, verify again, and finally delete the old index.
  • not intended to have the power of a full elasticsearch management interface -- just enough visibility to see whether things are going ok and where to start looking if something goes wrong
• for testing, support indexStrategy query param to /api/v2/search/creativeworks/_search, /api/feeds/rss, /api/feeds/atom
  • may request a configured strategy (e.g. indexStrategy=sharev2_elastic8) or a specific version of an index within a strategy (e.g. indexStrategy=sharev2_elastic8__bcaa90e8fa8a772580040a8edbedb5f727202d1fca20866948bc0eb0e935e51f)
• add FeatureFlag model, use it to switch default search strategy (name="elastic_eight_default")

TODO:

• [x] better unit-test coverage

-- cos jira ticket: ENG-4378

[coveralls]

Coverage: 91.835% (+7.3%) from 84.496% when pulling e538987c326a0eeb2766afa6fd278f35708732ce on aaxelb:quest/es8 into 21cd19c6c16216f6ec08876bcb83d6c645abdcdb on CenterForOpenScience:develop.

[coveralls]

Coverage: 83.229% (-1.3%) from 84.496% when pulling ccf2904adae75b862b6c416bb2e923fc54b38ca4 on aaxelb:quest/es8 into 21cd19c6c16216f6ec08876bcb83d6c645abdcdb on CenterForOpenScience:develop.