templates: spacing around Files section

[tiborsimko]

Records having directly attached files are not featuring enough spacing before the Files section heading, for example see https://opendata.cern.ch/record/7088

Note also the the Files heading is printed as "h3", whilst the other headings as "h2" usually on that page. Note also that "h3" is apparently printed in a bigger font than "h2", whilst it should usually be the other way round.

An easy solution could be to switch Files heading to "h3", and to rebalance the font sizes for h1/h2/h3 headings, however it may be good to profit from the occasion to more generally check the spacing before/after headings of each section displayed on the detailed record page. Perhaps we may have another field of this kind.