Implement "service preference" page

[michaelrhanson]

Either as a preference pane, or as about:social, we should have a place where the user can see all of the available providers.

This display should include, for each provider:

• name, icon, and domain name
• whether it is currently active, and a button to activate/deactivate
• whether it will be activated at browser startup
• whether notifications from the provider are allowed
• perhaps the most recent notifications? do we store them for a while?
• a way to delete the provider from the list

[mixedpuppy]

in the activities addon I have a very raw enable/disable page, it could be moved to this pretty easily

[mixedpuppy]

moved to https://bugzilla.mozilla.org/show_bug.cgi?id=755126