Implement administrator view

[jvoisin]

An administrator should be able to:

• Add rules
• Delete rules
• Add rulessets
• Delete rulesets

A user should be able to:

• View the rules
• Play with the sandbox

Shall we use flask-admin or is this useless and overkill?

• Implement an administrative view
• Implement two-factor authentication
• Review the permission model

[mdeous]

I think it could be useful, even more once we'll have implemented user accounts. If we chose to do this, Flask-admin should definitely be the way to do it.

I can take this task if needed, as I'm already quite familiar with this.