Add "Helper" Slug to Normandy Check Function for v2 Study

[maxxcrawford]

Fixed #209 - Added helper slug to checkNormandyAddonStudy function, s…o that users from that study are not thrown an error

Testing Steps

• Start with clean profile on unsigned latest build
• Install the Normandy Dev Tools
• Once installed, clicked the Wrench icon
• In the Normandy Dev Tools, run this script via "Write & Run Arbitrary" to enroll in v2 Helper Study
• Restart browser
• Verify you have the add-on via about:support
• Verify you are enrolled in the study via about:studies
• Depending on your testing scenario, add the following prefs:
  • doh-rollout.enabled : true
  • doh-rollout.debug : true
• If you add doh-rollout.enabled:true;, you will get the Doorhanger (if heuristics allow it)
• Close browser
• Delete doh-rollout extension from /extensions directory in profile files.
• Open browser, remove/reset pref doh-rollout.enabled entirely.
• Run this add-on (via web-ext) in the correct version of FF and correct profile.
• Navigate to about:debugging#/runtime/this-firefox, click "Inspect" button on the DoH Rollout add-on
• In that console, you should see the following:
  • Note: The values will be different based on if the add-on turned DoH on/off, but regardless, if the add-on ran, you will have sort of objects returned from the migrateLocalStoragePrefs function.

    Object { context: "migration", item: "doneFirstRun", value: true }
    Object { context: "migration", item: "skipHeuristicsCheck", value: false }
    Object { context: "migration", item: "doh-rollout.previous.trr.mode", value: 2 }
    Object { context: "migration", item: "doh-rollout.doorhanger-shown", value: true }
    Object { context: "migration", item: "doh-rollout.doorhanger-decision", value: "UIOk" }
    Object { context: "migration", item: "doh-rollout.disable-heuristics", value: undefined } 

Additional directions for testing (along with expected responses) can be found here.