Create a new admin panel

[fbaig]

Is your feature request related to a problem? Please describe.
No.

Describe the solution you'd like
An admin panel option in the top right dropdown menu. It should only be shown to users having role<=2

Additional context
The admin panel will have following functionalities to start with

• List all users, change users' role
• List elements with no related elements

[erickzli]

@fbaig We need to implement new endpoints for the above functions.

[erickzli]

Endpoint candidates:

• Listing all users: GET /api/users
• Change user role: PUT /api/uses/{user-id}/role with the new role in the JSON body.