Add Editor.js as a block-style editor

[wmnnd]

Editor.js should be added as an alternative way of creating campaigns.

The following blocks need to be available:

• [X] Paragraph
• [X] Headings
• [x] Nested Lists
• [x] Image
  • [x] Upload image
  • [x] Select previously uploaded image
  • [x] Alt text, title + caption
• [x] Spacer
• [x] Button
• [x] Columns
  • [x] 2-column layout
  • [x] 3-colum layout

The following inline formatters should be available:

• [x] bold
• [x] italic
• [x] underlined
• [ ] strike-through
• [x] link
• [ ] code
• [ ] Liquid