support string values ('true' and 'false') for configuring the secure parameter on external storage backends

fixes #78

some more reuse for fileactions

Fix user webdavauth

Checkstyle fixes 

also fixes an issue where some fileactions always worked on the last file in the list

LDAP: clear the cache not only when TTL changes, but with every settings...

I think Checkstyle did not find this occurence, however consistant line endings are preferred.
@DeepDiver can Checkstyle find this?

Checkstyle fixes 

Just one CSRF token per session

Checkstyle: many fixes

Files: change wording 'From url' to 'From link'