Proper offline support (file picker)

Expected behaviour

If I have files (currently about 10gigs of “favourite” scientific article and book PDFs) stored offline, I should be able to access them from the iOS file chooser when offline. This is especially important when proper iOS11 Files support is implemented (#268) so that the file picker actually becomes useful and usable. (See e.g. the “PDF Viewer” app that already uses iOS11 files support to not require internal storage—unfortunate works offline only with iCloud, but not with Dropbox or nextCloud that demand internet access to even open the file picker.)

Actual behaviour

Cannot even view directory structure when offline, let alone open stores files.

Steps to reproduce

iOS version

App version

Server configuration

Operating system:

Web server:

Database:

PHP version:

Nextcloud version: (see Nextcloud admin page)

No files, tests, or entry points are named. Start by reviewing issue #268 and reproducing the iOS file-picker behavior while offline. Done means the directory structure and locally stored PDFs can be accessed through the iOS file chooser without network access.