Refactor NewFeatureDialog to only show once on form landing page

[LMNTL]

Checklist

1. If you've added code that should be tested, add tests
2. If you've changed APIs, update (or create!) the documentation
3. Ensure the tests pass
4. Make sure that your code lints and that you've followed our coding style
5. Write a title and, if necessary, a description of your work suitable for publishing in our release notes
6. Mention any related issues in this repository (as #ISSUE) and in other repositories (as kobotoolbox/other#ISSUE)
7. Open an issue in the docs if there are UI/UX changes

Description

Makes the NewFeatureDialog for Anonymous Submissions only show once in the form landing page, instead of appearing next to the Enketo links and in the sharing modal.

Notes

• Makes the NewFeatureDialog component wrap its children, so that it doesn't need to be rendered inside of the component it highlights.
• Adds a new prop, featureKey, to NewFeatureDialog. Use it to differentiate between dialogs for different features.