Write old mdi.html exactly as hass.io expects (#1213)

* Write old mdi.html exactly as hass.io expects

* Create build dir if it doesn't exist

script/develop

@@ -4,10 +4,11 @@
 # Stop on errors
 set -e
 
+BUILD_DIR=build
 OUTPUT_DIR=hass_frontend
 OUTPUT_DIR_ES5=hass_frontend_es5
 
-rm -rf $OUTPUT_DIR $OUTPUT_DIR_ES5
+rm -rf $OUTPUT_DIR $OUTPUT_DIR_ES5 $BUILD_DIR
 cp -r public $OUTPUT_DIR
 mkdir $OUTPUT_DIR_ES5
 # Needed in case frontend repo installed with pip3 install -e