[ui] add ui update form route

[signebedi]

Child of #30, #116, and #42. Pull the user document and pass it as an HTML-friendly dict to the Jinja template. Unset values should be nulled in the HTML - so no default values should be used. We can pass the whole document to the API route and document database; only the changes will be captured, notwithstanding some funky data mutation stemming from type conversion.

[signebedi]

It's a legitimate question: do we plan to pass the existing data to the html generator function, or is the javascript going to do that? I think the javascript is going to need to do that, and route everything through the API routes (eg. pull the data and overwrite the default values). Otherwise, we are breaking core design assumptions that the UI will be built atop a fully Restful application...