Make registration receipt prettier

[willgearty]

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).

• Integrate with themed pages
• Make only part of the template configurable on the receipts page (use a block or insert context?)
• A button to use javascript to print only the receipt like in Add button to scheduler to print matrix #3469
• A button to go back to the registration page
• Student schedule should be formatted like in Better schedule styling for continued classes #3376
• complete dates for the program (although this is already in the schedule)
• the date that (class) registration closes? ("You have until X/Y/Z to change your classes")
• items purchased and/or amount owed?

Fixes #1131.

[willgearty]

Screenshot of new receipt with default pretext:

[kkbrum]

Looks great! I did not look into the test for you, but the functionality is all very nice!