Fix Solutions i18n issues

@chrisndodge @muhhshoaib @martynjames : this fixes some i18n problems introduced in 
https://github.com/edx/edx-platform/commit/2cc0433833bee4ccfb4955bfdbbff0edc363920a

We need to be really careful with i18n, it causes me a lot of headache when stuff is improperly marked up for i18n, because I have to drop everything, deal with translators, and make PRs to fix strings. Please read through https://github.com/edx/edx-platform/blob/master/docs/en_us/developers/source/i18n.rst and ask me if you have any questions.
This is a really minor nitpick, but ideally this would be indented as follows:
```
${_("You can no longer access this course because payment has not yet been received. You
can {link_start}contact the account holder{link_end} to request payment, or you can
{unenroll_link_start}unenroll{unenroll_link_end} from this course").format(
    link_start='<a href="#">',
    link_end='</a>',
    unenroll_link_start='<a id="unregister_block_course" rel="leanModal" data-course-id="${course.id | h}" data-course-number="${course.number | h}" href="#unenroll-modal">',
    unenroll_link_end="</a>"
)}
```