TextField and SelectField's shared FormField wrapper had no width of
its own, so inside the form's shrink-to-fit flex column each field's
input/select resolved its 'width: 100%' against its own shrunk
wrapper rather than a shared column width — two fields with
differently-long labels ended up with differently-wide controls.
FormField now caps its own width the same way the control already
does, so every field in a form lines up regardless of label length.
Also wrapped the page in a max-width container: Panel has no width
opinion of its own, so it filled the full page column, leaving a lot
of bare panel to the right of the ~16em-wide form.
f14fc154cc