Extract locale data

[muffinresearch]

Fixes mozilla/addons#9620

• Uses sprintf-js for placeholder subs
• I forked babel-gettext-extractor and patched it for the following:
  • 🐃 Fix for undefined translation keys which is what caused the empty output see muffinresearch/babel-gettext-extractor@5499190
  • 🐃 Fix for lack of content output idempotency. This is done by key sorting (which is a bit of a hack) but looks to work in practice. See muffinresearch/babel-gettext-extractor@f0f00b2
• Made the creation date dynamic
• Used a babel plugin for dedenting template literals so it gets pre-processed and is therefore correct in the translation.

Next up building some locales with the templates and using them...

Obviously we'll need to clean up and decide how to proceed wrt the plugin fork.

[coveralls]

Coverage remained the same at 100.0% when pulling 5655226 on muffinresearch:extract-pot into 2dbe4b7 on mozilla:master.

[mstriemer]

That's awesome that this string isn't in the disco one 👍

[mstriemer]

r+wc here. I'm going to look over the patches to the extractor too but we could merge those later if I have any comments.

[coveralls]

Coverage remained the same at 100.0% when pulling 558ce49 on muffinresearch:extract-pot into 2dbe4b7 on mozilla:master.