Generate and use "cluster" Conduit API tokens

Summary:
Ref T5955. Ref T2783.

  - Removes the "temporary" type. I was going to use this for T3628 but it started taking more time than I wanted to spend on it.
  - Add a "cluster" type, which is an internal-only token type used within a cluster. This token value is never shown to the user.
  - Automatically generate, use, and cycle cluster tokens.

Test Plan:
  - Diffusion (mostly) works with a repository configured to use a remote service.
  - Saw cluster tokens generate; terminated a cluster token and saw it regenerate.
  - Viewed cluster token in settings panel and saw nice explanatory text instead, as expected (we might just hide these eventually).

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2783, T5955

Differential Revision: https://secure.phabricator.com/D10990