Add filter expression explores that use a small, recent sample of data

[fbertsch]

With explore filters enabled via #97, we can now get filter suggestions for fields in Looker. The problem is the queries to get the data for these filters are something like:

SELECT $field
FROM mozdata.org_mozilla_firefox.metrics
WHERE DATE(submission_timestamp) >= '2010-01-01'

And they take upwards of 30s to complete. We should create a parallel explore just for filter suggestions, and use a few recent days and a sample of data to fill-in those filter values. These should load quickly for users.

These new explores would be created side-by-side our existing ones, with a default name - something like ${name_of_explore}_filter_suggestions, and it should be hidden: yes by default. The view would be able to figure out the name, and would load in the suggest_explore and suggest_dimension from that new explore name.

Alternatively, the view could generate the name, and send it over to the explore via configuration in namespaces.yaml, though that sounds more complicated.