Update mockups

[jasmussen]

These mockup updates include general refinements and polish, per Github and Slack discussions. Notably around:

• Galleries. Props @j-falk for ideas here. Trash and edit buttons when selecting individual images in a gallery, and button for advanced gallery editing, where you'd also be able to add more images to the gallery, or rearrange, in the case of a slideshow version of the gallery.
• Drag and drop. Props @iseulde for the idea of showing a blue line for the dropzone. The gray area is a ghost of where you're dragging from, so the blocks don't size around when you drag.
• A new approach to inserting content. It generally happens on a newline, and the plus then shows up to the left, where the up/down arrows would show up for a non-empty block. We'd then have a permanent "insert" button somewhere in the Admin UI to go with this, for when you need to insert content between two letters. See also Fitting image to whole view makes arrows disappear #78.
• Added empty versions of Quote and Image block. Imagine the use case of type type type, press enter for newline, type /ima to open inserter searched to Image, press enter, arrow down, type type type. In that case you'd simply inserted an image placeholder block which you could later upload to, or pick an image from the media library.