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.

• 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).