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.
@sarina got it. @chrisndodge, although unrelated to i18n, this filename is strange - why not `account_creation_and_enroll_email_message.txt`?