This enforces proper types on POST and GET arguments where I considered it sensible. I didn't update some as I don't know what kind of values they would support 🙈
Fixes https://github.com/owncloud/core/issues/14196 for core
Otherwise when the file without any specified mimetype was accessed the error log was flooded with entries such as "Undefined index: mime", there can be multiple issues found about this in the forum and our bugtracker.
To test this access `/index.php/apps/files/ajax/mimeicon.php` with and without `$_GET['mime']`.
Fixes itself.
maintain deprecated \OC::$session when getting or setting the session via the server container or UserSession
restore order os OC::$session and OC::$CLI
remove unneded initialization of dummy session
write back session when $useCustomSession is true
log warning when deprecated app is used
