implement on the run caffeine submission

- caffeine.forms.SubmitCaffeineForm:
  - return caffeine instance from save method
- caffeine.urls:
  - add URL pattern submit_caffeine_otr for
    caffeine.views.SubmitCaffeineOnTheRunView
- caffeine.views:
  - extract base class BaseSubmitCaffeineView from SubmitCaffeineView
  - restrict supported HTTP methods of BaseSubmitCaffeineView to allow post
    only
  - add success message in BaseSubmitCaffeineView.form_valid
  - drop unneeded BaseSubmitCaffeineView.post
  - add new SubmitCaffeineOnTheRunView based on BaseSubmitCaffeineView and
    getting user information from URL variables
- add templates/ontherun.html with the on the run form buttons