Retrieve Group Wiki data after trial ends or tier is downgraded

Release notes

Problem to solve

Group-level Wikis are available on Premium/Silver and higher plans. If a user downgrades or completes their trial period without purchasing Premium or Ultimate, they lose access to any data stored in a group wiki.

Proposal

The data for the group wiki is stored in a separate git repo. This is still on disk after the trial period or downgrade is complete. So, we should provide read-only access to the group wiki repository data through the API, allowing users to package and download it from the command line.

But why not do this in the UI

There are several reasons we would not want to provide read-only access to the group wiki data in the UI. But the primary reason is time investment. This MVC allows access to user data without investing in branching front-end logic and different experiences based on a user's current tier. The proposal to download a wiki repository from the API actually has utility in itself as well.

In the future, based on demand, we could build on this and provide a way to complete the download through the UI, so this iteration makes the most sense as the smallest possible change to solve the problem.

Documentation

This will require step-by-step documentation in the Wiki documentation.