Feature-gating for end of census

Today's TSM reminded me that at the end of the Census, we'll want to turn off some features. Specifically:
- New account registration
- Event creation
- Tree mapping

Those features should be designed with this in mind.

We'll also need to update the copy on the homepage once the census is over.

Start by locating the code paths for new account registration, event creation, tree mapping, and the homepage copy. Determine how each feature can be disabled after the census, then verify that all three are unavailable and the homepage text reflects the post-census state.

Written by the indexing model from the issue text.