UI Rule Editing: Only allow valid solr fields

[linuxswords]

Currently it is possible to add any string as a solr field for a rule.

This is problematic if it does not match a field declaration in solr, resulting in errors querying solr.

Although there is a popup with suggestions but than can be ignored and the entry edited

Proposal is that the entry is validated against the existing solr fields. It should not be allowed to store invalid fields.