cleanup: use student_continue and student_finished partials

DO NOT MERGE UNTIL AFTER CSEDWEEK (HAML CLEANUP)

As evidenced by the dead learn2 link change earlier today, we had 3 places in the tree that were using copied blocks of more or less the same html.  I moved these to a partial.  The prizes UX used in a slightly different way, so there is a second partial that is optionally included by the first partial.

Note: this does impact the hero HTML that is pulled in by the top-level code.org page.  The partial uses what was on that hero page (which included some extra IDs and style names) to avoid any visible changes on the code.org page.