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.
Would recommend using `start_tag` instead of `tag` for (a) consistency with edx-platform and (b) to clearly delineate which tag should come first.