Create fake test data while using the app

When testing the app it would be useful to be able to generate fake contacts, groups and messages quickly.

• Add a contact (saving the identity private key somewhere so we can create signed messages later)
• Add 100 private messages to/from a fake contact
• Add 100 posts to a forum (nested replies from a mixture of throwaway fake identities, fake contacts and the user's own identity)
• Add 100 posts to a blog (comment chains from a mixture of throwaway fake identities, fake contacts and the user's own identity - these could be posted to the blogs of fake contacts or the user's own blog)

Alternatively, instead of creating the messages in batches, we could start a background service that generates them at random intervals until shutdown. That would also allow us to test notifications, UI updates, etc.