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))
Since #2573 landed we are no longer using `@import` in stylesheets (other than lib.less, which is just mixins). Can you please add this to the bundle in base.py instead? Thanks @kyoshino