Bug 37040: Prevent ErrorDocument subrequests from activating CSRF

Bug 37040: Prevent ErrorDocument subrequests from activating CSRF

This change improves the mechanism for preventing the CSRF middleware
being activated by ErrorDocument subrequests.

This change was necessary due to a subtle issue identified by
Bug 37041.

Test plan:
0. Apply the patch
1. Restart Koha
koha-plack --restart kohadev
2. Go to http://localhost:8081/cgi-bin/koha/cataloguing/addbiblio.pl?biblionumber=9908
3. Log in
4. Note that you get a pretty 403 and not an ugly plain text error
5. Go to http://localhost:8081
6. Fill in the login details, but use the HTML inspector to delete
the csrf_token from the hidden inputs
7. Submit the login
8. Note a pretty 403 page

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Matt Blenkinsop <matt.blenkinsop@ptfs-europe.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>