Stop recording assignments for feature gates

[jmileham]

(equivalent of https://github.com/Betterment/test_track_rails_client/issues/43)

Per this comment on test_track server: https://github.com/Betterment/test_track/pull/69#issuecomment-380995245

Since gates (splits having a name with a suffix of _enabled) are not used for analysis, we can save storage and increase the fluidity of release/rollback operations by not storing assignments at all for feature gates. If we plan not to store them, then we can also skip sending segment and testtrack assignment events.

A byproduct of not storing them is an increased amount of assignment event chattiness from clients that will never realize that a visitor has already been assigned a variant for a given split. This will be a fast event for TestTrack to serve because it will quickly realize that there's no work to do, but we might as well kill the event sending from the client and avoid confusion about duplicate assignment events.