Overide default draft-js handling of pasting text/html
This is surprisingly needed to avoid an issue with draft-js that causes multi-line madness when pasting code and then applying format-as-code to it - https://github.com/vector-im/riot-web/issues/2120#issuecomment-271735729.
The issue sounds like it is https://github.com/facebook/draft-js/issues/170#issuecomment-195026203 and the suggstion is to override the text pasting handler https://github.com/facebook/draft-js/issues/170#issuecomment-215983216. Meanwhile they haven't modified the default pasting behaviour afaics.
I've discovered a separate issue that is apparent even after this suggested fix. (https://github.com/vector-im/riot-web/issues/4446)
