Build: Fix the ESLint config for demos

The ESLint config for `demos` extends the one for the `ui` directory. However,
the `ui` one used to not be a root one, making the `demos` one effectively not
depend on the `jquery` preset. Fix that and fix lots of lint violations in
`demos/search.js`.

Closes gh-2303

demos/.eslintrc.json

@@ -1,5 +1,9 @@
 {
 	"root": true,
 
-	"extends": "../ui/.eslintrc.json"
+	"extends": "../ui/.eslintrc.json",
+
+	"globals": {
+		"require": true
+	}
 }

ui/.eslintrc.json

@@ -1,4 +1,8 @@
 {
+	"root": true,
+
+	"extends": "jquery",
+
 	"parserOptions": {
 		"ecmaVersion": 5
 	},