EnrolmentUF object can be changed by a ManyToMany relationship

[emieza]

• [x] In order to enable an student to select their UFs, the EnrolmentUF object works, but it would be more easy to integrate and use a ManyToMany relationship between Enrolment and UF object.

[rubenmantecon]

Should be done now https://github.com/AWS2/djmatrics/commit/4ab191dcb40a0f90dffb7e90837b085d6e4dacf5 @emieza