Add generic time JS function, update listings and include all listings as per the spec

app/admin/routes.py

app/bookings/routes.py

app/main/utils.py

app/models/listing_images.py

app/models/listings.py

app/static/generic.js

app/templates/admin/create_listing.html

app/templates/admin/edit_booking.html

app/templates/base.html

...ds/listing_images/17_2d852af9f5b64b1abdd5784e8172ce6e.gif

...ds/listing_images/22_200629e3a75949aabbe9b800fa7f91db.gif