Clean-up endpoints for Equipment & Gym management

Closes #3 (closed)

General Details:

• Currently we will only support creating a gym from there backened. Time permitting we can add screens to allow users in-app to create gyms too.
• Ensure each route other than auth routes are "protected". Refer to Ktor sessions authentication. You can add the following at the beginning of every route: val userSession = call.sessions.get<UserSession>()
• By the end of this ticket, we should be able to manage users, manage gym (CRUD), manage equipment within the gym (CRUD)

Endpoints to be completed:

gyms: GET: /gyms POST: /gyms PUT: /gyms/{id} DELETE: /gyms/{id}

gymAdmins: GET: /gyms/{gym_id}/admins POST: /gyms/{gym_id}/admins PUT: /gyms/{gym_id}/admin/{admin_id} DELETE: /gyms/{gym_id}/admin/{admin_id}

exercises: GET: gyms/{id}/exercises POST: gyms/{id]/exercises PUT: gyms/{id}/exercises/{id} DELETE: gyms/{id}/exercises/{id}