remove boxy styling from definitions

style it more like an exercise / as in mock-up / as in cnx
make "copy element" option less conspicuous
improve styling of x and gear

make them lighter since i apparently can't make them thinner :-(
better spacing (centered and without extra space above/below)
hover behavior more like mock-up
update styling of quotations

use left/right quote mark images instead of generic box to style
quotations
remove "extra" padding around figures
transition height for solution-controls too ...

... so that exercises don't take up as much space when not hovered
style multiparts like other exercises

i.e. without the boxiness
hide "click to add part" on mouseleave

space-saving measure same as it's done for regular exercises
make blue droppable area less tall

plus a little margin so it doesn't look too awkwardly close to the
elements it'll be between.
remove excessive and redundant min-heights
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.
make drag handle more truly centered
explaining this use of important

there's gotta be a better way
Merge pull request #89 from oerpub/58657378-boxes-and-spacing-2nd-attempt

58657378 boxes and spacing 2nd attempt