share content using system share sheet

[jpwiedekopf]

This commit fixes #2 by removing the WRITE_EXTERNAL_STORAGE permission (which isn't granted on Android 13 and up!), and may not pass app review for the GPS.

Instead, it uses a FileProvider to provide content URIs for a file written to the internal storage of the app, and provides READ permission to the receiving app. This commit was tested against Android 13, with more tests to follow.

[jpwiedekopf]

Definition of done:

• [x] Prototype for sharing tested on Android 13
• [x] Manifest permission removed from app
• [x] Sharing via content URIs implemented
• [x] old code removed
• [x] verified instrumented tests don't fail
• [x] facility for migrating config versions implemented/prepared
• [x] sharing into the app implemented
• [x] tested on Android 5 (API 22) - if only one app is able to handle the transaction (GMail in the emulator) no share sheet pops up, but installing an Editor from FDroid made a picker show up.
• [x] tested on Android 7 (API 25) -
• [x] tested on Android 9 (API 28) - shows that no apps can handle by default, but FDroid fix worked well
• [x] tested on Android 11 (API 30) - works great.
• [x] tested on Android 13 (API 33)- works great; also tested on bare metal (Pixel 6).

[jpwiedekopf]

also closes #4.