This patch moves us from using an old pref `general.useragent.locale`combined
with `intl.locale.matchOS` for retrieving user requested locale, to use a new
preference `intl.locale.requested` which stores a list of well-formed BCP47
language tags. If set to empty, the OS locales are used. If not set at all,
default locale is used.
We are also adding a piece of code to migrate from old to new system.
MozReview-Commit-ID: 854yQ1kC6Ee
--HG--
extra : rebase_source : c4a7171bc026f857f7878ee83d973ec01b536a84