Updated blocktrans strings for i18n

One case in particular:
Due <span> [date] (in [time]) </span>
Was not going to be possible to remove from the translator's view.  In place of that removal we added a helpful comment to translators.
`num_to_be_graded` perhaps instead of `must_grade`? If you don't know how ORA works, `must_grade` is confusing.