Show "TBD/TBA" locations to enable early, "save the date"-like announcements

Can we modify event_digest.html so that it falls back to whichever free-text the event organizer added after the campus: field, in case the campii still needs to be decided? The code for lecturer seems to do that, but @schlauch & me weren't sure how to adapt that templating code.

Alternatively, a TBD campus could be added, however, sometimes at least the city or region can be added, like DLR would like to do in !159 (merged), !163 (merged) & !164 (merged). For example, the latter's result like this would be helpful to learners already: