Bug 969101: Initial implementation of a first-time use ping. r=fabrice

v1.3 uplift
https://bugzilla.mozilla.org/show_bug.cgi?id=969101

Cherry picked from my 1.4 commit. The only non-minor conflict was the necessary use of `requireApp` in the FTU ping unit test, which passes locally. Using this function in master caused TBPL to fail, so I have a scheduled run here to make sure there is no bustage:

https://tbpl.mozilla.org/?tree=Try&rev=7a16ab65335e