Dedup fields into generalized search field.

[4ndygu]

not ready yet, but this is mostly for visibility's sake.

This PR adds a new search field to the elasticsearch backend that consists of all previously queryable fields, joined into a single space-delimited field. This allows us to cut a lot of nested logic from the search parameter and avoid the complexity that deals with search queries that span multiple nested field (e.g. wanting a search that looks for an OR relationship between one opinion, but also another case title).

the costs of the current implementation below are:

• repeated calls of the prepare_%s functions
• ugliness of the way fields are currently added into the filter_set variable

attached is a successful test deploy of a query in both the case name and in the case body text.

[codecov]

Codecov Report

Merging #2033 (9f44628) into develop (02a8c16) will decrease coverage by 0.24%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #2033      +/-   ##
===========================================
- Coverage    62.94%   62.69%   -0.25%     
===========================================
  Files          103      103              
  Lines        11488    11493       +5     
===========================================
- Hits          7231     7206      -25     
- Misses        4257     4287      +30     

Impacted Files	Coverage Δ
capstone/capstone/capapi/filters.py	79.38% <0.00%> (-7.01%)	⬇️
capstone/capstone/capapi/serializers.py	92.85% <0.00%> (-2.11%)	⬇️
capstone/capstone/capapi/documents.py	91.07% <0.00%> (+0.33%)	⬆️

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