Langpack auto updater for daily builds

[whimboo]

Originally filed as: https://bugzilla.mozilla.org/show_bug.cgi?id=706418

User Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:11.0a1) Gecko/20111128 Firefox/11.0a1
Build ID: 20111128031052

Steps to reproduce:

Update or only download langpack xpi (force x32 if x64 doesn't exist) for checking or testing compatibility.
Eventually, on start, disable langpack on errors.

[xabolcs]

With this gist somebody could easily install / update a langpack.
Another essential part of the fix is the logic which assembles langpackURL.

A long-duration timer will be needed to check for updates. IMHO check interval must be at least 1 day.
Of course a button to trigger manually the update check install of latest version would be nice.

Checking for errors on startup isn't trivial, so I suggest to disable a locale if it got incompatible (e.g. due to an uplift).
This gist lists all the "foreign" locales and it's compatibility status.

[MikkCZ]

I guess this is not possible with WebExtensions anymore, and not needed when all languages should have their Nightly builds available. Also new strings are currently qurantined and not exposed to localizers for few days after they are added to mozilla-central, so the errors would be pretty commons I think.