Move contracts to a new Contract model

[polypixeldev]

Summary of the problem

Currently, contracts are in an OrganizerPosition::Contract model and tightly coupled to OrganizerPositionInvite. With the new application flow I'm working on, we'll need contracts to be abstracted more to support a source from either an Event::Application or OrganizerPositionInvite, since OrganizerPositionInvite will not exist yet when the applicant is signing the contract during the application flow.

Describe your changes

Primarily, this PR adds a new Contract model that is mostly the same as OrganizerPosition::Contract, with a couple differences:

• Has a polymorphic relationship Contractable instead of a relationship directly to OrganizerPositionInvite. Only OrganizerPositionInvite implements this right now, but in the applications PR Event::Application will also implement it.
• A type column instead of a purpose column. The purpose column was added in the original implementation of OrganizerPosition::Contract, but it is not currently being used. This PR adds this back as STI, and a future PR will implement better automation for termination contracts

All code that references OrganizerPosition::Contract has been changed to reference Contract instead, including the relations on User and Event. A future PR will drop the OrganizerPosition::Contract model and associated table.

In the database migration that creates this new table, steps have been added that copy over current data from the organizer_position_contracts table and renames item types for versions stored in the PaperTrail versions table.

I've tested this PR using my own DocuSeal account in test mode, but it should be tested with our production DocuSeal account and a real contract before merging into production.

[sampoder]

lgtm! minus comment on type of contract

[garyhtou]

Good schema!