Bugfix: Do not load files from cache

When apps (i.e Drawio) tried to load a file, the browser caches the request.
If the file was modified somewhere else and the browser reads the file and its
version from the cache, the content shown to the user is outdated and saving any
changes is impossible until the cache is properly cleared.
Thus we now ask the browser to never load files from its cache in apps.

In order to achieve that we send a `Cache-Control` header along with requests.
Unfortunately currently released ownCloud 10 versions do not accept that header
in cross site origin setups. If you run ownCloud Web on a different domain than
your ownCloud 10 instance, then you might need to add `Cache-Control` to the list
of allowed CORS headers:

`occ config:system:set cors.allowed-headers --type json --value '["cache-control"]'`

Please make sure you don't override previous values!


https://github.com/owncloud/web/pull/6447
https://github.com/owncloud/core/pull/40024
