[#691] Remove fq on legacy search API and force public datasets

Even if it is a backwards breaking change we sholdn't be allowing the fq
parameter to be set through the API as it can lead to privacy issues. We
will also enforce that all datasets available through the API are public
(as the v3 API does)
[#691] Add note to legacy search docs about private datasets
Merge pull request #691 from okfn/691-override-fq-on-legacy-api

Disallow fq parameter on the legacy search API and force to search only public datasets