Improve the URL builder

[rowanmanning]

Depending on how many people actually use it (we should check Splunk logs to get an idea before investing time), there are some improvements we could make to the URL builder:

• Make the form work client-side so that the image updates automatically
• Add validation to the form so that you can see what the error is if you've made a mistake
• Write some integration tests for it (eep!)