Refactor username update to work with new profile API

[vnugent]

Important: New user signup is temporarily disabled to prevent introducing consistent data

What would you like to be able to do? Please describe.

I'm introducing a new user profile API in the backend to incrementally migrate user profile data to our own db. For this issue, create a new username update screen that uses the new get/set username API.

Related:

• Username Validation Incorrect Warning #718
• When editing username, if you try to go back to your old username it says it is already taken #575
• Resolving User fields within the GQL schema openbeta-graphql#202

How important is this to you (Please pick one)

• Important

Todos

• Create a new M2M action to add custom claim uuid
• Create a new get access token API
• Create a new username update screen