Fix side panel search filter

[didoda]

This fixes search from side panel with advanced filters by adjusting properly the schema and using, if available, a /:type instead of /objects?filter[type][]=:type, when there is only one related type for relation.

This fixes the form, by using proper schema; in the following example, the custom field "booking status" is an enum, and it is shown as a combo; without this fix, it is shown as an input text field.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.64%. Comparing base (62e2b3c) to head (70415af).
Report is 30 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1212      +/-   ##
============================================
- Coverage     99.65%   99.64%   -0.01%     
+ Complexity     1326     1316      -10     
============================================
  Files            98       97       -1     
  Lines          5205     5121      -84     
============================================
- Hits           5187     5103      -84     
  Misses           18       18              

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