Add Aragon Agreements

[lkngtn]

Is your feature request related to a problem? Please describe.
When Aragon Agreements launches it will provide a way for a Garden to be created with an initial idea or purpose that defines the purpose of and identity of a community. Agreements will be implemented as a forwarder, so we can assign specific roles to it which will require actions to be delayed for a period of time allowing them to be challenged.

Describe the solution you'd like

• Replace Toll Gate with Agreement
• Assign Remove proposal role to Agreement (related to Onboarding: deployment script gardens#3)