Fix Bug 1103396 - Move Thunderbird start page to bedrock and make it localizable

Applied the Sky theme. ([screenshot](https://bugzilla.mozilla.org/attachment.cgi?id=8528032))
I've been through and tested a fair amount of bedrocks supported locales for this particular link, and the ones I've tried do indeed seem to redirect to en-US correctly. That's not to say I can rule out the possibility of errors, like in the link above (but I believe this is not a valid lang format, right?)

@flodolo - what are your thoughts here. Do you think this is ok, or something we should probably avoid crossing fingers on?