Editor: Suggest codeblock or citation if user pasted some content

[sebix]

When a user pastes some text (more than a few words/lines), the editor could right away suggest the user wrap the pasted text in a citation or code block. This would be very handy for all new users and could avoid lots of edits by supporters and moderators.

JavaScript features an onpaste event.

[chris34]

'Suggest codeblock' could be also related to #294