feat: sql handlers for custom media types

* test text/html and drop HtmlRawOutputSpec.hs
* all tests passing, removed all pendingWith
* make functions compatible with pg <= 12
* move custom media types tests to own spec
* anyelement aggregate
* apply aggregates without a final function
* overriding works
* overriding anyelement with particular agg
* cannot override vendored media types
* plan spec works with custom aggregate
* renamed media types to make clear which ones are overridable
* correct content negotiation with same weight
* text/tab-separated-values media type
* text/csv with BOM plus content-disposition header

src/PostgREST/Error.hs

@@ -91,7 +91,7 @@ instance PgrstError ApiRequestError where
   status SingularityError{}      = HTTP.status406
   status PGRSTParseError         = HTTP.status500
 
-  headers SingularityError{}     = [MediaType.toContentType $ MTSingularJSON False]
+  headers SingularityError{}     = [MediaType.toContentType $ MTVndSingularJSON False]
   headers _ = mempty
 
 toJsonPgrstError :: ErrorCode -> Text -> Maybe JSON.Value -> Maybe JSON.Value -> JSON.Value