WebHost: Add robots.txt to WebHost (#3157)

* Add a `robots.txt` file to prevent crawlers from scraping the site

* Added `ASSET_RIGHTS` entry to config.yaml to control whether `/robots.txt` is served or not

* Always import robots.py, determine config in route function

* Finish writing a comment

* Remove unnecessary redundant import and config

WebHostLib/robots.py

@@ -0,0 +1,14 @@
+from WebHostLib import app
+from flask import abort
+from . import cache
+
+
+@cache.cached()
+@app.route('/robots.txt')
+def robots():
+    # If this host is not official, do not allow search engine crawling
+    if not app.config["ASSET_RIGHTS"]:
+        return app.send_static_file('robots.txt')
+
+    # Send 404 if the host has affirmed this to be the official WebHost
+    abort(404)