Redirections

First part of solution to #77.

~~Since there are lot of changes in the same places as in confirmation_email branch I've based redirections branch on confirmation_email and commits from Jul 19, 2013 (first 8 commits) are from confirmation_email branch.~~

1. Unified `action in ...` assertions
   * Added `ACTIONS` tuple to utils.py.
   * Used `utils.ACTIONS` in views.py, models.py, test_maling.py.

2. Refactored views
   * Added `ProcessUrlDataMixin` that processes args and kwargs from url before before dispatching request.
   * Added `ActionMixin` mixin retrieving action and adding it to context.
   * Added `ActionTemplateView` that renders a template for proper action, with newsletter and action in context.
  * Added `ActionFormView` FormView with newsletter and action support.
  * Adapted views to inheritance from above mixins/base views.

3. Added "success" pages
   * Added urls and templates with info about sent email.
   * Added urls and templates with info about activation success.
   * Add tests for new pages.

4. Added redirections for anonymous user's actions
   * Added redirection after activation email is sent.
   * Added redirection after action is successfully activated.
   * Added redirection after action request for switched off email confirmation.
   * Adapted templates.
   * Adapted tests.