Add an admin panel site

[Shirajuki]

The admin panel site should be a place where admins can add, edit and delete events and users.
Some minimum requirements for the admin panel:

• Only admins should be able to view the admin panel
• The admin panel should have a search, sort and filter functionality
• The admin panel should list all members with all fields
• The admin panel should be able to edit members (group membership, all attributes, etc)

Nice to have:

• History on all edits and changes
• Draft view on events

This is a large task, so tasks are broken down to smaller issues:

• Admin panel site: events #21
• Admin panel site: users #22
• Admin panel site: membership #23