'Rest' tab improvement: submit request on ENTER.

In the current version, in the **rest** tab, the user has click on the "send" button to submit a request. Also, depending on your screen size, the "send" button might not even be above the fold, so you would also have to scroll down to find it, and that's for each and every request.

This PR improves the overall user experience by adding a `<form>` element to the template and also making sure only one button is of type `submit`, thus allowing the user to submit a request by pressing ENTER when any input element has focus.

Obs: the diff is **much** easier to read if you [tell GitHub to ignore whitespace](214/files?w=1).