Allow agreements to be fully customizable

Currently we only allow URLs for agreements. To comply with the MLH Member Agreement, members are required to phrase agreements in a certain format. This format is currently not supported with the new agreement model. 

Reference: https://docs.google.com/document/d/1K7HSIEO8tA7vbD0dtwvesMOhDAea8nDKJlu3f3kS8pE/edit