Support channel-izing the CRLite filters

[jcjones]

trafficstars

We might want to provide the same data in one format for Firefox X and a different format for Firefox X+1, or perhaps one for Release and one for Beta. (But more likely cut on version numbers). We should figure out how to delineate them for that purpose.

[jcjones]

The mechanism is to set a field on the object named filter_expression, which obeys the rules at https://remote-settings.readthedocs.io/en/latest/target-filters.html