Review and improve Slotting UX

[zippy]

• Currently when dragging an item to be slotted, the coloring of the spaces that match turn: fully green, partially yellow and not at all red depending on the level of match. I'm guessing the UX could be improved...
• When dragging an item if you drag over a different item and drop you get a color change on the item to indicate merge possibility. This UX could probably be improved.
• Unslotting is just dragging back to the orphans column. Doesn't look great.
• Clear slot button (Reset) in session update window. Doesn't look great.