feat: enroll in multiple semesters

[david-shortman]

Allows parents to enroll for classes across multiple active semesters.

It'll only take four hours 🙃

For parents:

• shows class selection within new semester tabs (#61)
• shows classes by semester
  • on enrollment confirmation step
  • on enrollment success email
  • on account historical enrollments page

For admins:

• provides drop-downs to view by semester for
  • dashboard
  • enrollments
  • t-shirts
  • class forms (#87)

Under-the-hood:

• adjusts the following to look up classes by semester:
  • cost & discount calculation
  • processing an enrollment
  • showing a parent's enrollment history
  • generating an "enrollment success" email

Also:

• checked on how discounts applied
• updated 2023 language
• removed requirement to have credit card when $0 cost
• message on signing every semester