Add field for optional edit summary

[lethosor]

This would probably be appended onto the default summary (which could be an issue with the 255-character limit)

[lethosor]

One way around the limit could be abbreviating the summary (e.g. {Q12}) and expanding the text with a hook when displayed. There doesn't appear to be a hook that does this, so it may have to be implemented at the HTML rendering stage. Hopefully there's a less ugly solution.