fix(client): Firefox for Android Sync marketing material is only available to desktop sync users.

fixes #1041
Refactor the marketing snippet logic.

* Extract the marketing logic from `ready.js` to `marketing-snippet.js`
* English speakers always see some form of marketing.
* If an english speaking sync on desktop user is not selected to see the survey, they will see the `Get Firefox for Android` marketing.
* Non-english speaking sync on desktop users always see the `Get Firefox for Android` marketing.
Merge pull request #1299 from mozilla/issue-1041-hide-sync-for-most

fix(client): Firefox for Android Sync marketing material is only available to desktop sync users.