Add `FundingOpportunity`

Inherits from `News` and defines:

funder | Agent | 0-n |   | The entity funding the opportunity.
-- | -- | -- | -- | --
type | String | 0-n |   |  
opening_date | datetime | 0-1 |   | The start date of the application for the Fundind Opportunity.
closing_date | datetime | 0-1 |   | The end date of the application for the Fundind Opportunity.
minimum_contribution | decimal | 0-1 |   | The lowest amount of funding that can be requested or granted for the opportunity.
maximum_contribution | decimal | 0-1 |   | The highest amount of funding that can be requested or granted for the opportunity.
announcement_page | URL | 0-1 |   | The location where sites the original announcement.
submission_platform | URL | 0-1 |   | The location where sites the platform for the submission.