merchant can edit products

@cgruppioni @aslawter 

This branch was repurposed after it was created to focusing on editing as a whole. The branch name was changed locally, but I haven't been able to do anything about the name here.
Casey, the code doesn't work if it's only `<% render "form %>`

I took the `form_for` out of the partial, so there needs to be one outside of it.