User should be able to rescan/validate user data if they choose

[holmesworcester]

In #409 we are choosing to ship Zbay with data from included channels (users, channel of channel, price oracle, default channels) so that we can start the lightwallet with the birthdate of the app release.

In #290 we let users rescan to see old messages.

With this issue we want to give users a way to rescan the data that Zbay provided them to validate it. (At some point in the future we will do this automatically by default, and warn users if there is a discrepancy, but that's something for future work once we figure out a general solution to "going back in time".)

Ideas:

• Tell user in the startup slides
• Tell user in a welcome message in the Zbay channel
• Tell them in a full screen modal after they register their username (rescan, or continue)
• Include a section / button in the security section that lets people rescan. This section would change once they rescanned.

Basic idea for text:

"To start quickly, Zbay includes existing username and channel registrations in the app itself. If you'd like to restart and verify these, click here. It will take ~15 minutes. "

UPDATE: see my comment below for instructions on completing this.

[holmesworcester]

One thing is the padding and vertical rhythm on the security page is getting messy. I wonder how many more sections we can sustain. Probably it's best to put the "Outbound links" section last since it has subsections of indefinite length.

[holmesworcester]

1. Add this welcome text to the message people get when they join the Zbay channel for the first time:

Welcome to Zbay! To start quickly, Zbay includes username and channel registration data in the app itself. To verify this data, which takes ~1 hour but may add some security, restart & re-sync. Otherwise, say hi and introduce yourself!

2. Add this section to the security settings.

Verification

To start quickly, Zbay includes username and channel registration data in the app itself. If the Zbay team omitted some data, or provided incorrect data, a user or channel could be censored or impersonated. To verify this data, which takes ~1 hour but may add some security, you can restart & re-sync. You only need to do this once.

BUTTON: Restart & re-sync

Note: once the user has done this, we should say "Zbay has been re-synced."

3. Rescan all username and channel data whenever user rescans (e.g. when rescanning an old channel) and throw away any data we've shipped with the app, and any data based on it.

4. Put the "outbound links" section at the bottom, and the recovery key section at the top, followed by the verification section.

[holmesworcester]

Note: we are already showing the union of messages included with the app and messages from the node, on rescan. This is important for security.