Show database stats and export options in Settings sheet

[AngeloStavrow]

To help investigate issues like #200 —and to provide some user-facing data— it could be helpful to have a "Local Data" section in the Settings sheet (iOS) and/or Preferences window (macOS) that:

• displays the total number of blogs/collections stored on the local device
• displays the total number of posts stored on the local device, broken down by
  • post status (LOCAL, EDITED, PUBLISHED)
  • post type (draft/anonymous, vs. part of a blog)
• an export feature that allows the user to export all collection data and all post data as a pair of JSON files

@thebaer Thoughts?