Restructure /mf2/ permalinks for citability

To make the URLs more citable in other forms i.e. Twitter, it'd be more useful to have them as shorter URLs, as well as including some information about the date, so they're easier to sort / determine when they are from.

This requires amending our existing /mf2/ entries, which involves a few more changes in some files than others due to it being done with the Ruby pretty-printer opposed to the Java one.

Closes #702 (closed).