Create relationship between climbing coalition and membership application

[Syntaf]

Description

At the moment, membership applications only include a string field referencing the name of the coalition. This should be refactored to instead hold a reference to the climbing coalition itself for better data consistency.

Acceptance Criteria

• Database migration created to remove string column and add reference column
• JointMembershipApplication contains a belongs_to association to LocalClimbingOrg
• Membership signup form refactored to support the above association
• Tests pass