Use paths, not query strings

[mnot]

Right now everything is shoved into query strings on /, which is not great.

[mnot]

Straw-man:

• / - the current main form
• /check/{uri} - run test (POST to run, GET to get the form)
• /saved/{id} - saved results (POST to extend, GET to return)
• /client-error - client error (POST)
• /slack - various slack things

Probably easiest to start with client-error and check. Saved results will need some continuity; as may slack.