Updating .dashicons class to match core
Updated arrows, added new code icon (unif505), added move (unif504), opposite-of-distraction-free (unif506), props @melchoyce, @empireoflight
Replaced old code icon (unif475)
Accessibility icon updated

unif483 no longer has the circle around it. In case we need it again, I
copied it to unif507
Adding new dashicons classes

.dashicons-editor-contract, .dashicons-external,
.dashicons-accessibility-alt
Merge branch 'master' into gh-pages