backend/auth: redo OAuth flow and "finish" DM auth flow

rpc/auth.js

@@ -0,0 +1,16 @@
+// @flow
+import { type AppContext } from '../Roleypoly'
+import { type Context } from 'koa'
+
+export default ($: AppContext) => ({
+  async checkAuthChallenge (ctx: Context, text: string): Promise<boolean> {
+    const chall = await $.auth.fetchDMChallenge({ human: text })
+    if (chall == null) {
+      return false
+    }
+
+    $.auth.injectSessionFromChallenge(ctx, chall)
+    $.auth.deleteDMChallenge(chall)
+    return true
+  }
+})