Save additional user preferences

[matthew-white]

Now that we have the ability to save user preferences (see #689), we can start saving more of them. This issue is to keep track of places where we want to save additional user preferences.

Top-level/sitewide preferences:

• Whether the top panel on the homepage is shown or not

Project preferences:

• (?) The sort order of the form list
  • It's not clear we want this. We've talked about giving project managers the ability to customize the order of forms in the form list.
• (?) Whether managed or legacy QR codes are shown for app users
  • Are we planning to offer legacy QR codes indefinitely? Or will that go away once we add the ability to configure QR codes?