Various permission-related fixes (#114)

* Do not unnecessarily ask for storage write permission on Android 11

* Fix download PDF not working via HTTP on recent Android versions

* Add missing READ_EXTERNAL_STORAGE permission to manifest

* Reword storage permission request message in intro

* Fix strings wrongly marked as non-translatable

* Handle missing READ_EXTERNAL_STORAGE permission when opening file

* Tweak action_about Italian string to better distinguish it from settings

* Use correct icon for document info dialog

* Remove non-existing WRITE_INTERNAL_STORAGE permission from manifest