Move security headers to base.php

Some headers were currently only added to the templates but not to other components (e.g. SabreDAV / JSON / etc...)
The migration to base.php ensures that the headers are served to all requests passing base.php
Fix indentation
Remove uneeded usages of nosniff
Typo + Line breaks
Fix typo

Thanks @DeepDiver1975
Merge pull request #8183 from owncloud/move-security-headers

Move security headers