Add a `stakeholders` field to projects to relate it to profiles

[jackbravo]

Add a stakeholders field to projects to relate it to profiles.

For now, it can be only a simple related field.

Alternatively We could just add stakeholder to the disciplines catalogue that we use for the Looking for field and we will start using for the ProjectMembers role with #348 .

Future tasks:

• Allow any user to suggest stakeholders (SMEs) from a list (https://airtable.com/shraXiAYzQ4EGafum/tblGtPiry38Kh0Tmf) depending on the impacted area, and then the person needs to accept being a stakeholder.
• A person can either accept his nomination as a stakeholder, or admins would be able to also accept or directly add a stakeholder to a project