fix cloze+latex issue in media check, and duplicate answers (#757, #758)

we were replacing spans with spaces during review but not during media check.
just strip the tags instead, which makes the media check consistent and
doesn't result in multiple identical answers
make sure to update order when adding/deleting/changing option groups (#675)
safe mode won't work on 4.7
add mature today stat
prioritize html over images to fix MS issue (#449)

this means that copying&pasting images on linux/firefox will redownload the
image which is wasteful, but Windows users make up the majority of the
userbase
ignore _ in field references (#803)