implement CITP Search Study consent, closes #760

Checklist for reviewer:

[x] The description should reference a bug or github issue, if relevant.
[x] There must be a CHANGELOG.md entry for any non-test change.
[x] Any change to the NPM commands must be carefully reviewed to make sure it won't break the Add-ons pipeline.
[x] Any version increase must follow the release process.

mozilla-rally / rally-core-addon