[ui] Allow prefilling form fields using URL params

[signebedi]

If URL params are passed on the /create endpoint for a given form, we should try to capture these and set them as the "default" value...

[signebedi]

SCOPE: We've applied this feature to the create route, but not to duplicate or update because doing so runs contrary to the specific goals of those two separate routes. The engineering overextension that this might beget is that the generateLookup() functions are different in, on the one hand, the create jinja2 template, and, on the other, the duplicate and update routes, the latter of which include no logic to pull URL params.