Implement ooniauth API v2

[hellais]

This adds a new endpoint which implements a slightly nicer ooniauth API.

The main changes are the following:

• POST /v1/user_register -> POST /v2/ooniauth/user-login
• GET /v1/user_login -> POST /v2/ooniauth/user-session with login_token in the request body
• GET /v1/user_refresh_token -> POST /v2/ooniauth/user-session with authentication header set
• GET /_/account_metadata -> GET /v2/ooniauth/user-session

So basically instead of 4 endpoints, we now have only 2:

• /v2/ooniauth/user-login (implements only POST)
• /v2/ooniauth/user-session (implements both POST and GET). The POST accepts a token in body the json body or the authorization header. Token in the request body takes priority over the authorization header.

[hellais]

I will soon deploy this to dev and update this to dev and provide a link to the docs for the API.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (0c9bdd9) to head (a47beec).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #825 +/- ## =========================================== + Coverage 99.75% 100.00% +0.24% =========================================== Files 19 21 +2 Lines 1243 1439 +196 Branches 103 116 +13 =========================================== + Hits 1240 1439 +199 + Misses 2 0 -2 + Partials 1 0 -1 ``` | [Flag](https://app.codecov.io/gh/ooni/backend/pull/825/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ooni) | Coverage Δ | | |---|---|---| | [ooniauth](https://app.codecov.io/gh/ooni/backend/pull/825/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ooni) | `100.00% <100.00%> (+0.75%)` | :arrow_up: | | [oonirun](https://app.codecov.io/gh/ooni/backend/pull/825/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ooni) | `100.00% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ooni#carryforward-flags-in-the-pull-request-comment) to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.