Add block to page (inline)

[brynwhyman]

User Story

As a CMS user, I want to be able to add new content blocks within a page level between current content blocks, so that I can do this quickly and in context with the other page blocks.

Acceptance Criteria

• Hover-over between current page blocks presents styling as per wireframe: https://projects.invisionapp.com/share/ZRL4T2HQAGB#/screens/301918525
• Clicking a “+” button in between blocks presents a pop-up with all blocks available in a format that’s easy to view
• Clicking a new block type adds the block to the present gap between blocks. Styling animation pushes current page blocks below new block as per wireframe: https://projects.invisionapp.com/share/ZRL4T2HQAGB#/screens/301918523
• As per designs, when hovering between blocks there is a indicator that a block can be added between/above/below existing blocks (delayed interaction so the line isn’t triggered as you move the cursor across the screen). https://invis.io/BYMQXG07QZH#/307488192_Adding_Blocks-_In-Page

Notes

• Pop-up work is also covered in: Add block to page via pop-up  #293

[NightJar]

Done #443
in tandem with #340