Stripe subscription management

[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 description of your work suitable for publishing on our forum
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

Changes the redirects after Stripe sessions to point to the proper URL. Add 'Manage' button to the currently subscribed plan, which redirects to the Stripe customer portal. Moves the "Your Plan" display based on the logged-in user's current plan.

Related issues