Fix to allow javascript catch the right CSRF_COOKIE_NAME. Issue #1713

* Added javascript global variable `__csrf_cookie_name__`, it will
be displayed only in admin views and when the toolbar is available
so it doesn't expose this variable in the public version of the pages
 * Fixed the javascript code where the 'csrftoken' string is hardcoded.
 * Modified views and templates to pass to the context the
CSRF_COOKIE_NAME and use it to set the value of the global javascript
variable `window.__csrf_cookie_name__` falling back to the default
'csrftoken' value

The solution is not very clean, this is open to suggestions to improve the code and also how to test this functionality.

The commit in this PR can also be cherry picked on top of "develop" branch, as the same problem exists there.