Proversity/fix checkout currency

Fix to show correct currency specified in PAID_COURSE_REGISTRATION_CURRENCY setting on the "You are upgrading your enrollment for" (verify_student/upgrade) page. USD was hard-coded in there. Respects use of HTML entity values for currency symbol (eg. &pound. Could have used the course mode currency but that is code only (no symbol). Let me know if I need to write any tests for this. R