Filtering on Painless scripted field throws error

[Bargs]

We temporarily disabled the scripted field filter tests so we could update the esvm snapshots, we'll need to revert that PR and have the tests go green before we can release 6.1: #15169

I've already confirmed with the ES team that this is a bug in ES and will be fixed in 6.1 (filed in elastic/elasticsearch#26760). I'm creating this ticket as a reminder for us to test in Kibana once the fix has landed in ES.

Kibana version: 7.0, 6.1

Elasticsearch version: 7.0, 6.1

Description of the problem including expected versus actual behavior:

Attemping to filter on a Painless scripted field pops up a notifier error saying "Variable [doc] is not defined."

Steps to reproduce:

1. Create a Painless scripted field
2. Go to Discover
3. Attempt to create a filter on your scripted field

[rjernst]

Note that as discussed and agreed to in elastic/elasticsearch#26429, the "fix" is only a temporary measure to add back the existing hack for expressions in filter scripts. Kibana should switch to use painless for filters by 7.0.