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.
Show a proper error message when trying to scan the filesystem for a non existing user
Merge pull request #11225 from owncloud/showLoginAgainInsteadOfJSON

Show login again instead of JSON if CSRF check fails
Merge pull request #11281 from owncloud/scanner-unkown-user

Show a proper error message when trying to scan the filesystem for a non...
Clarify possible preview providers for type Office

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Disable external code coverage
Merge pull request #11287 from owncloud/kill-with

Disable external code coverage
Merge pull request #11286 from owncloud/clarify-possible-formats

Clarify possible preview providers for type Office
[tx-robot] updated from transifex
Expose creatable permissions trough fileinfo
Fix dav permissions for folders without create permissions