Merge pull request #9490 from jedp/863417-prefs-for-persona-urls

Bug 863417 - Can now set toolkit.identity.uri as pref; r=fabrice, a=tef+(cherry picked from commit 9dbb2d9ee919ea75d33be495ff710b7c3a3b24d9)

Bug 863417 - pref for identity ui, fix line too long; a=tef+
(cherry picked from commit f48d569231583b91118f1c9c848796259e35aa7b)