fix: make the rql completion working again

similar to cubicweb/cubicweb!158

the json route has been removed from cubicweb (was deprecated a long time ago).
the ajax route must be used now.
4 jobs for topic/default/fix-completion in 1 minute and 23 seconds (queued for 24 seconds)
Status Name Job ID Coverage
  Lint
passed check-manifest #183815

00:00:20

passed flake8 #183814

00:00:19

passed yamllint #183816

00:00:18

 
  Tests
passed py3 #183817

00:01:03