apply.html: scroll to the stall type when it changes

In order to see the rest of the form when it appears, use the
JavaScript obj.scrollIntoView(true) method to scroll it to the
top of the browser window when the stall type changes.
online applications: replace n_spaces with mc_stall_option

The Market & Crafts stalls now follow a different scheme, with
either indoors or outdoors options.

* extra/models.py: add mc_stall_option field to extra_stallevent
  table which is an enumeration of the 3 possible options
* extra/forms.py: replace n_spaces with mc_stall_option in
  StallForm as this is now the new way to define the stall set-up
* extra/public/apply.py: show the stall option instead of the
  number of spaces in the confirmation e-mail
* templates/public/apply.html: tweak text, prices and replace
  spaces with stall options
* templates/public/thank-you-email.html: replace n_spaces with
  mc_stall_option
* templates/public/thank-you-email.txt: fix sender name at the
  bottom of the email