Provide new “Token Usage” page (for admins)

[fpetersen-gl]

Allows Graylog admins to review all configured tokens on a single page (which they should do on a regular basis, especially after changes in their user base).

Shows a table with the following columns:

• User (link to the User page)
• Token Name (link to the User Tokens page)
• Token Creation Date
• Token Last Used At
• External User yes/no
• Name of auth backend (if external user)
  • or (DELETED), if auth backend config no longer exists in MongoDB
• Include a button to delete token.

We place this new page Under the System > Users