OAuth migration | Use new set username route

[raphaelkabo]

What does this change?

As part of the OAuth migration, we've removed an old bug in IDAPI - being able to set your username more than once, if you happened to know the API endpoint to hit (because setting username just used POST /user/me). We've already introduced a new IDAPI endpoint, POST /user/me/username, which only allows setting username once and only if it's not been set before. Once OAuth authorization to IDAPI is enabled in MMA, we will also be preventing the username being changed via POST /user/me. This PR modifies the MMA frontend to use the new endpoint, and adds an E2E test for username updating.

Dependencies

• https://github.com/guardian/identity/pull/2492, because we need to let IDAPI create test users without a preset username before the E2E tests here pass.

Tests

• Tested on CODE

[pvighi]

looks good to me

[prout-bot]

Seen on PROD (merged by @raphaelkabo 10 minutes and 39 seconds ago) Please check your changes!