Setup addons-frontend in pontoon

[muffinresearch]

Once mozilla/addons-frontend#423 lands we should be ready for the l10ners to set up this project in pontoon.

[muffinresearch]

@mathjazz has set this up in stage already https://mozilla-pontoon-staging.herokuapp.com/sl/amo-frontend/LC_MESSAGES/disco.po/?string=160086

[muffinresearch]

Checking if there's more to do...

[mathjazz]

There actually is one thing - @mozilla-pontoon needs to be added as a collaborator to this repo in order to be able to push translations to it.

[andymckay]

Done.

[mathjazz]

Confirmed in mozilla/addons-frontend@b97779d. Thanks!

[muffinresearch]

Thanks @mathjazz