Bug 1518632: Show the first-run experience for users that get pushed to a new profile. r=Gijs, r=flod, r=stomlinson

On startup of a fresh dedicated profile show a welcome page and a modal dialog
to explain what has happened.

--HG--
extra : rebase_source : 1505cf27f900070debc1f9e1c71ec4bef3bc099d
extra : source : 05200c5388b4f7adc4414268727458515d7b9ed9