[WIP] Richer admin UI.

Some things on our radar involve an admin UI that is smarter than what settings provides us so far. For example, *if* we implement weights, that UI will need to be different a way I'm not sure can be expressed with just our current settings UI. Group reset is also beyond the settings UI.

This approach might be kind of radical. It lets any plugin register panels on the admin screen, as well as run arbitrary scripts on the admin screen. This is also kind of a sketch of what client side plugins might look like in the future. Oh, and I brought in Nunjucks.

[x] Draw settings UI as plugin-driven panels.
[x] Add behavior with plugin-driven admin scripts.
[x] Make settings work for scalar values.
[ ] Make settings work for array values. (Currently draws, but does not update).
[ ] Make the message log a plugin.
[ ] Make command and control a plugin.
[ ] Move groups back to the main admin screen?
[ ] Tweak the UI as needed.