feat(healthcheck): add /_health endpoint

This endpoint returns the internal readiness state used by
the realtime code to indicate whether HedgeDoc is performing
properly. As it only returns the state of a variable, it is
less resource hungry compared to a call to /status for
checking the health of HedgeDoc.

By prepending the route with an underscore, it should not be conflicting with already created pads in FreeURL mode.

Signed-off-by: Erik Michelson <github@erik.michelson.eu>

lib/web/statusRouter.js

@@ -12,6 +12,16 @@ const { urlencodedParser } = require('./utils')
 
 const statusRouter = module.exports = Router()
 
+statusRouter.get('/_health', function (req, res) {
+  res.set({
+    'Cache-Control': 'private', // only cache by client
+    'X-Robots-Tag': 'noindex, nofollow' // prevent crawling
+  })
+  res.send({
+    ready: !realtime.maintenance
+  })
+})
+
 // get status
 statusRouter.get('/status', function (req, res, next) {
   realtime.getStatus(function (data) {