Optional fellowship switch

[f-gate]

solves: #192

API changes

You now have to pass in wether you need the applicant to be a Role::Freelancer.
It will check pallet_fellowship to see wether this is the case.

pub fn create_brief(
            origin: OriginFor<T>,
            mut brief_owners: BoundedBriefOwners<T>,
            applicant: AccountIdOf<T>,
            budget: BalanceOf<T>,
            initial_contribution: BalanceOf<T>,
            brief_id: BriefHash,
            currency_id: CurrencyId,
            milestones: BoundedProposedMilestones<T>,
            require_fellowship: bool,
        ) -> DispatchResult

Migrations

No migrations needed, since applicant is checked on creation there is no need to save it to the brief type.