Specify units for media offset and media playback in storypins

[jonpmarino]

thanks to #1520 we have ability to autoplay video media in storypins. There is also an ability to define how long a video plays, and where it starts. We should clarify for the user the units used in those options though (i.e. is 120 equal to 2 minutes or 120 minutes?). Just some quick text (like 'units in seconds') would suffice.