Make registration receipt prettier

This makes the student registration receipt prettier (like the PDF student schedules). The receipt can be seen by clicking the "confirm registration" button. The "pretext" can be edited by changing the confirm receipt on the settings page (and the entire receipt can be overriden by a single template override or by program-specific template overrides, like before).

[x] Integrate with themed pages
[x] Make only part of the template configurable on the receipts page (use a block or insert context?)
[x] A button to use javascript to print only the receipt like in https://github.com/learning-unlimited/ESP-Website/pull/3469
[x] A button to go back to the registration page
[x] Student schedule should be formatted like in https://github.com/learning-unlimited/ESP-Website/pull/3376
[x] complete dates for the program (although this is already in the schedule)
[x] the date that (class) registration closes? ("You have until X/Y/Z to change your classes")
[x] items purchased and/or amount owed?

Fixes #1131.

learning-unlimited / ESP-Website

Make registration receipt prettier #3706