fix default course mode and checkout for variable default currency

Includes fix to default course mode for variable PAID_COURSE_REGISTRATION_CURRENCY and checkout templates to use the setting on the "You are upgrading your enrollment for" (verify_student/upgrade) pages. USD was hard-coded. Respects use of HTML entity values for currency symbol (eg. £). 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.

Also there were 2 meta definitions in the CourseMode class. Combined them.