Do not create new quotes on enter

[psypersky]

If you create a quotation and then click 'enter' it will create another one, im new to the codebase so suggestions of where to look in the code to fix it are welcome :)

[rishijain]

The way it works right now is if you press enter, it does create a new blockquote, but if you press backspace upon an empty blockquote, it changes to p tag.

You can find most of the handling of formatting around blockquote here https://github.com/yabwe/medium-editor/blob/master/dist/js/medium-editor.js#L920

[nmielnik]

Fixed via #1162