Make DDO the primary homepage

[toolness]

Users who did HP Action can't send a LoC from the Take Action page because the current LoC splash page redirects the user to the latest step of their signup intent (e.g., HP Action) if they're already logged in.

We also want to make the root ("homepage") of the site be data-driven onboarding (DDO), not the LoC splash page.

This fixes the above issues by adding an explicit LoC splash page at /loc/splash, and making the site root be DDO. We also redirect the old /ddo to the site root, passing along the search params as needed.

To do

• Once Add link to LoC and HP action from login page #888 is merged, merge master back into this PR and make sure the LoC link on the login page points to the splash page.
• Make the homepage DDO.
• Redirect DDO to the homepage, so that old/bookmarked links pointing to DDO still work.
• Ensure that when users log in, we don't redirect them to a blank search screen. And that if they click on the JustFix logo, it does the same thing as clicking "Take Action" on the navbar.

[toolness]

Note that once #888 is merged, we'll want to replace the LoC link there too!

[toolness]

[ch414]