feat(#439): Open attachments on mobile systems

It is possible to attach files to items in the app since a long time. Opening an attachment basically just opens that file in the local file system.

However, on mobile systems (Android and iOS), this didn't work - because obviously these files are located in the app's private folder and hence other apps cannot access them. This is changed now by introducing a new ShareUtils class, which abstracts away opening such local files and provides a working flow on mobile OSes.

Closes #439 (closed)