Fix sign-in while running a server from amo:dev

[kumar303]

Fixes mozilla/addons#10667

This does a few things:

• Both amo:dev and amo:stage go through the proxy which means both the frontend site and the backend API are served on the same domain like production. This should help catch same origin issues locally.
• Sign-ins work on amo:dev
• Sign-ins do not work on amo:stage (like before) because we need to configure a new FxA client. I'll do that in a follow-up.

[willdurand]

🎉 💯

Just tested this PR and it works well. Thanks @kumar303. It also works for newly created accounts (which did not work before), except that the user gets redirected to http://localhost:3000/en-US/firefox/users/edit, which does not exit (404). I dont think it is in the scope of this PR though. Oh and logout works too.

[kumar303]

@willdurand great, thanks for testing it out. I am reminded now that the redirection 404 (for new accounts) is a known issue that will be fixed when mozilla/addons#10588 lands. However, with my patch you will be logged in when you type in the URL localhost:3000 after the 404 so it's not too annoying.

[tofumatt]

Just testing this now, I was beat up by a migraine most of today, sorry! 😢

[tofumatt]

Just tested locally and I could also sign in and sign out. Looks good, thank you! 👍

[tofumatt]

Shoot! My bad, I think this change was my fault 😢

[kumar303]

btw, I also tested add-on/theme installation through a custom Nightly build

[kumar303]

@willdurand be sure to clear your cookies when switching between yarn amo:dev, yarn amo, and yarn amo:stage since they all share the same domain (localhost:3000).

[willdurand]

ah good to know, thanks!