theme_boost: Remove caption-specific styling and use bootstrap default styles

For the 3.2 release, we've decided to overwrite the Bootstrap 4 default positioning for captions, using a 'caption-side: top' css instead of the BS default, 'caption-side: bottom'. This was done to maintain the look and feel from 3.1->3.2 as the clean theme had them top-positioned.

Part of the problem with bottom positioning, is that some of our captions have been styled to looks like headings, with large font, heading-like lang strings ending with colons (e.g. 'Some table: ') etc, so when using the bootstrap default (bottom), this just looked wrong.

This issue should remove any custom styling for captions, allowing them to be bottom positioned without looking out of place.

A few places that need to be looked at:
competencies, calendar, charts and activity results block. Note the results block uses a colon at the end of the lang string, so this will need to be changed too.