Sidenav and path changes for activity feature

[brrkrmn]

Summary

This PR addresses the changes mentioned in #1099.

Closes #1099

Changes

• Create activity button is visible to 'educator' and 'staff' role users
• Creators get a warning when they try to visit /activities/create
• Explore Activities button in the sidenav redirect the users to /activities

Screenshots

For Educators:

For Creators:

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