[WIP] Remove loadAppScriptFile

:warning: This is WIP do not merge

This removes `loadAppScriptFile` and enforces the usage of proper routing.

Fixes https://github.com/owncloud/core/issues/9060

# Todo

- [ ] core
- [x] files
- [x] files_encryption
- [ ] files_sharing
- [ ] files_trashbin
- [ ] files_versions
- [x] user_ldap
- [x] user_webdavauth
- [ ] Send mail to developer ML
Drop the `@return void` or is this needed if nothing is returned?