assistant/init with exp_uid in route

[daniel3735928559]

• /assistant/init/<app_id>/<exp_uid> should provide a form pre-populated with the values used to start the experiment
• All submissions of the /assistant/init form, whether got using the pre-populated or blank version, should take you to /assistant/init/$the_app_id/$new_exp_uid where new_exp_uid is the ID of the experiment just started. This will allow easily starting up a fixed version of a just-started experiment that was started with an incorrect parameter.