FIX: Declare context definition to be JSON schema, fix validation errors

[effigies]

meta.context.contextmeta.context is a JSON schema document embedded in the BIDS schema. This establishes that in metaschema.json and resolves the bugs that would have been discovered sooner if we had done this.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.04%. Comparing base (8367dbe) to head (8f83656).
Report is 282 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1867   +/-   ##
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  Coverage   88.04%   88.04%           
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  Files          16       16           
  Lines        1380     1380           
=======================================
  Hits         1215     1215           
  Misses        165      165           

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[effigies]

@rwblair I think this should be a quick one.

[effigies]

One question: Do we want to consider removing the extra context: level, and just start with type: object? That would modify the schema structure, but not a lot depends on this yet, so it's probably not a terrible time to do it.

[rwblair]

Sounds good to me.