Add support for setting and returning account_id

[hellais]

@majakomel reported that we actually need the account_id for OONI Run to work.

We might eventually want to remove the redundancy by just storing email addresses directly.

[codecov[bot]]

Codecov Report

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

Project coverage is 100.00%. Comparing base (e486936) to head (3f407fc).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #828 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 21 21 Lines 1439 1448 +9 Branches 116 116 ========================================= + Hits 1439 1448 +9 ``` | [Flag](https://app.codecov.io/gh/ooni/backend/pull/828/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/828/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%> (ø)` | | | [oonirun](https://app.codecov.io/gh/ooni/backend/pull/828/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.