Bug 906580 - [email][l10n][meta] Support dynamic language changes by setting data-l10n-id and data-l10n-args on all dynamic nodes. r=asuth

With some minor lint/whitespace fixups by asuth as well as a cleanup in
folder_picker.js where it turned out we didn't need to be localizing
what we were localizing since we immediately clobbered it.

This will obsolete https://github.com/mozilla-b2g/gaia/pull/21614