attempt to solve slippery "add a new part" links

This is my attempt at solving the issue of having those "add a part" and
"add an answer/solution" links be so hard to click when they
disappear/reappear on hover, while still having
exercises/solutions/multiparts take up as little extraneous space as
possible.
Merge remote-tracking branch 'origin/58657378-boxes-and-spacing-2nd-attempt' into testing