refactor(campaign)!: remove refundable parameter

[k-yang]

Highly opinionated change.

Campaigns have this refundable field which is only checked in the cancel function. In order to cancel a campaign, the admin has to first call the set_refundable function to toggle the field to true. Once the field is true, it can never be flipped back to false.

The value of set_refundable: false does not provide any additional security guarantees because the admin of the contract or owner of the campaign can always call the set_refundable function to toggle it to true. Once set to true, it can never be flipped back to false.

However, having to set the refundable field to true manually by executing a separate function call is annoying and a waste of gas/compute. It would be simpler to have campaigns that are always cancellable/refundable.

BREAKING CHANGE: Remove refundable parameter from campaigns.