Quickfix for https://github.com/owncloud/core/issues/11064

* test case added to avoid adding them later

adding X-Robots-Tag to all responses of ownCloud + move addSecurityHeaders() to OC_Response, which seems to be a more reasonable place

- Use OCP\Response constants instead of the HTTP error code
- Use checkAppEnabled() instead of OC_App::isEnabled with an if statement
- Remove uneeded variable $baseURL
- Rename $isvalid to $isValid

The Freebox is the multimedia device of a french Internet provider: Free. This device provides a seedbox which uses the user agent "Mozilla/5.0". In the "Content-Disposition" header, if the "filename" key is used with the "filename*=UTF-8''" value, the seedbox does not take care about the header and saves the file name with the origin URL. This patch brings the support for the Freebox users.

- added isUserAgent() method to OC_Request which makes it possible to
  test it
- OC_Response::setContentDisposition now uses OC_Request::isUserAgent()

Fixes #5807

Added new OC\Response API called setContentDispositionHeader() that
contains the needed workarounds for UTF8 and IE.

Refactored download code to use the new API.

Removed unused trashbin download file.

Conflicts:
	lib/private/vcategories.php