ui/users: create user dialog

[dctalbot]

• Identified the issue which this PR solves.
• Read the CONTRIBUTING document.
• Code builds clean without any errors or warnings.
• Added appropriate tests for any new functionality.
• All new and existing tests passed.
• Added comments in the code, where necessary.
• Ran make check to catch common errors. Fixed any that came up.

Description:
Currently, goalert admins can only create users for basic auth via the CLI. This PR brings that functionality into the UI.

Part of #1247

Screenshots:

Describe any introduced user-facing changes:
Admins will have a create user fab on the users list page

Describe any introduced API changes:

  createUser(input: CreateUserInput!): User

input CreateUserInput {
  username: String!
  password: String!
  name: String
  email: String
  role: UserRole
}

Additional Info:

• This functionality may move into a central user management page in the future

[mastercactapus]

A couple of initial thoughts:

• For role, maybe just an Admin checkmark in the dialog to replicate the --admin flag in CLI rather than making it a multi-step process
• Still allow creating a user if Basic auth is disabled, but add a notice to the dialog that they will be unable to login until it's turned on

[Forfold]

Code looks good, but functionally I'm seeing field errors for username show up under the display name field instead

[dctalbot]

Code looks good, but functionally I'm seeing field errors for username show up under the display name field instead

Steps to reproduce? I'm not seeing that @Forfold

[Forfold]

For sure:

1. Set username to something "test " (note the space at the end)
2. Fill in rest of required fields
3. Attempt submit

[Forfold]

lgtm!