feat(ee): implement user authentication api

[darknight]

For closing #805

Changes include:

• Update GraphQL schema, add 4 mutations, generated schema looks like

type Mutation {
  register(
    email: String!
    username: String!
    password1: String!
    password2: String!
  ): RegisterResponse!
  tokenAuth(
    email: String
    username: String
    password: String!
  ): TokenAuthResponse!
  verifyToken(token: String!): VerifyTokenResponse!
}

The refreshToken implementation has not been finalized, so I do plan to raise separate PR for it.

• Update mutation resetRegistrationToken, only admin is allowed to execute it

There are some implementations need to be discussed, comment inline.

[wsxiaoys]

Thanks for being patient on this PR, LGTM