Option to store data inside or outside the app

[ajburley]

Allow the user to choose whether data is stored inside or outside the app.

• If inside the app, the encrypted data cannot easily be accessed by someone who gets access to the filesystem, such as another app. Also, the data is less prone to accidental deletion by users who like to clean up their storage by deleting things they don't recognise. Finally, if the app is uninstalled, the data will automatically be deleted.
• If outside the app, the user can use third-party applications to sync the data with some external system, for example DropSync. If the app is uninstalled, we have no way to delete the data, but some may see this as a good thing.

The pros and cons should be explained to the user in the settings screen. The default option should still be to store inside the app (as currently).

[ajburley]

Somewhat in preparation for this, one of the changes in 83f3a6667e8b03232fe6432baedd51e7b2e2b1af was to remove the text "access of notes directly from disk is not possible" from the README.