feature(website): Update AuthenticationManager to get user data from the GraphQL API

[gabrieldonadel]

Why

In order to support fetching more complex data involving user fields, we should use the GraphQL API instead of api/v2. Necessary for querying things like:

{
  me {
    id
    username 
    userExperiments {
      id
      enabled
      experiment
    }
  }
}

In a follow-up PR we should add something like graphql-tag to have more robust support for the GraphQL queries

How

Replace the existing userInfo request with an equivalent query using the GraphQL API

Test Plan

Run the website locally and ensure user data is loaded correctly

[gabrieldonadel]

Did you check if the other properties aren't used anywhere?

Yes, the only properties used were id, username and picture