Update Tanu-Key frontend to focus on project secrets only for the MVC

Based on the discussion here, we will be focusing on just project secrets for the Tanu-Key MVC. The frontend was set up to possibly support both group and project secrets, but group secrets (and inheritance) will now be a post-MVC feature to be developed later as we get more feedback and research around it.

This issue is for discussing and implementing changes to align the frontend efforts with this decision. The biggest change would probably be:

• Changing how the feature flag is used. Currently, we set the feature flag on the root ancestor (group) instead of projects.
• Hiding the routes for the group secrets page.