When the admin creates a new custom field they can specify if that field should be searchable or not.
That setting is taken into consideration for quick search results.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
Krzysztof Kotlarek
Sam Saffron
Thorben Egberts
Robin Ward
Neil Lalonde
Régis Hanol