improve registration_success page

[glbert-does]

the registration_success pages (template, view) could be improved by giving a summary of the options picked in the registration, maybe highlighting changes if the registration was updated.

i am marking this as "good first issue" because it sounds relatively easy, but i did not actually look at it much.

edit: highlighting changes is possibly a bit harder than i initially thought, but probably still doable. also not strictly necessary, just nice to have.