Refuse API writes that are not application/json (closes #271)

A page on another origin of the same site could send a text/plain or
form POST with the user's cookie, since a browser sends such a request
without a preflight. RequireJSON on the /api/v1 router answers 415 to a
POST, PUT or DELETE whose Content-Type is not application/json, or that
has none, before the handler runs.

The web client and neoirc-cli already send the header on every request.

Model: opus-5-5

README.md

@@ -521,7 +521,8 @@ for every HTTP request when metrics are on.
 Every other `GET` path serves the web client. A path that names one of its files
 gets that file; any other path gets its `index.html`. An unknown path under
 `/api/v1/` gets a plain-text `404`. A request whose method the path does not
-take gets `405` with no body.
+take gets `405` with no body. Under `/api/v1/`, a `POST`, `PUT` or `DELETE`
+without `Content-Type: application/json` gets `415` instead.
 
 ### The message envelope