Adds kramdown footnote link schema to Sanitize config

Kramdown footnote anchors are generated with id's containing colons, eg: 'fn:1' or 'fnref:1'

Sanitize is seeing those in 'a' tag hrefs as links with dodgy protocols (eg. href="javascript:kill_everthing()").
This commit adds the footnote id prefixes to Sanitize's list of allowed protocols to get round the
issue.

Relates to story: https://www.pivotaltracker.com/story/show/60399700