Deprecate NoRent

[shakao]

• adds an environment variable IS_NORENT_DEPRECATED that we can use to enable deprecation

when the environment flag is enabled it:

• adds a modal that appears on every NoRent page (copy still in progress)
• reroutes all the letter creation pages to the homepage
• adds a backend check to disable the letter sending function as well
• tests for the backend and frontend deprecation scenarios

[sc-11224]

[shortcut-integration[bot]]

This pull request has been linked to Shortcut Story #11224: NoRent deprecation.