Add editor UI with Medium-style controls

[thibaudcolas]

That is:

• No toolbar at the top of the editor. Instead,
• Controls on the sides of the editor to add block-level formats and content blocks.
• Inline "hover" toolbar to add inline formats and content entities.

[katie-day]

I love the medium editor, it's so great! The only thing I wonder is (I presume) a lot of our audience is used to seeing the toolbar at the top (word, google docs etc) so will going for a medium-esque style editor confuse the average user or it's 2017 and people should be used to things changing and improving!

[thibaudcolas]

There's now an example of this as part of #171.