Add admin-only views for creating and setting billing IDs

[matthew-li]

Changes

• Added an admin view (Admin > LBL Project IDs: Create button) for creating a billing ID, with logic identical to the create subcommand of billing_ids described in Enable listing of billing IDs from the command-line #559.
  • The view is only accessible to superusers.
• Added an admin view (Admin > LBL Project IDs: Pencil icon in individual table rows) for setting the billing ID for a particular Project, ProjectUser, or User, with logic nearly identical to the set subcommand of billing_ids described in Enable listing of billing IDs from the command-line #559.
  • The view is only accessible to superusers.
  • One difference in the logic:
    • If a Project or ProjectUser does not currently have a billing ID, one cannot be set from the UI, while it can from the CLI, so the latter must be used.
    • If a User does not currently have a billing ID, one can be set from either the UI or the CLI.