Show login again instead of JSON if CSRF check fails

Previously a JSON error page was shown to the user in-case the CSRF token was not valid. This was confusing and prevented people from login.

With this at least the login page is shown again and not a JSON error message. I consider this as sufficient since adding a new error page just for this sake would uneededly make lib/base.php even more cluttered and this is a edge-case which optimally should anyways not happen that often.

This can be tested by opening the login page, then clearing the cookies, and trying to login.
user/backed: use pow of two for backed action constants

the current implementation limits number of possible backed actions
to 8 as it uses pow of 16 for constants. This change introduces pow
of two and allows up-to 32 actions to be defined.

The old values are preserved for backward compatibility.
Drop only if exists
Merge pull request #11275 from owncloud/drop-if-e

Drop only if exists
Merge pull request #11229 from kofemann/for-upstream

user/backed: use pow of two for backed action constants
Merge pull request #11225 from owncloud/showLoginAgainInsteadOfJSON

Show login again instead of JSON if CSRF check fails