Allow urlencoded requests as well as JSON requests

urlencoded requests are easier to make from client-side JS,
and are the only option available to plain HTML forms.

Change-Id: I2c3a336b9fc6418890c7278550a7426427c8cde2
Add a test form to the welcome page

Change-Id: I47ca81267d11b776a8736bcf9cb339fb43df4873