Adds .bold and applies it to reference on hover

- Should probably only do this while lesson is open right now
since otherwise it looks like you can do something with it
Reference doesn't bold unless lesson is active

- With just the "X" it doesn't really show up much, the mouse
mostly covers it, need to find a better indicator
Quirks with reference bolding being worked out

It's a bit messy. I'm not sure why it stays bold after the
transition when it was hovered over. I don't like messy.