[Cashtab] Move directory out of web and to top level

Summary:
T3067

Cashtab is a large and complex application showcasing many features of other ecash dependencies and libraries. Though a web wallet, it isn't really a website. It's also more than just another app.

This diff moves Cashtab to its own top level directory as part of a planned monorepo directory organization process.

Test Plan:

• Confirm this diff alters no files in the cashtab/ directory
• Confirm .arclint is updated appropriately with new dir info
• Confirm build-configurations.yml is updated appropriately with new dir info
• Make sure CI still works on unit tests (call bot token)
• Make sure deployment paths are updated before landing

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13570