Make Ember admin work with new access control

[sebgie]

This belongs to the OAuth Epic: #4004 - please read this for the big picture of what this issue is for :)

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The changes for making OAuth work for external clients are going to cause some updates to how we use OAuth with Ember admin. Ghost admin is a special application as it is the only application that is allowed to use username and password to login directly using the Resource Owner Password Credentials Grant (RFC 6749 - 4.3).

To keep the Ember admin working correctly, we will need to update the values for new fields created in #4174:

• client_id: generated random number
• redirection_uri: null
• trusted_domains: null (if the client is of type Ghost Admin the url from config.js should be used)
• logo: Ghost admin logo
• status: enabled
• type: Ghost Admin
• description: tbd

In addition to that, the client_id needs to be available to the admin. This could be done similar to how we make fileStorage settings available. Authentication needs to be updated to use the new client_id as well.

[ErisDS]

This is already complete