[reledit] Fix reledit icons jumpiness (closes #4106867)
Introduce an "invisible" class (copied verbatim from Bootstrap) that
only sets the "visibility" property. "display: none" is the reason for
the whole jumpiness as it actually detaches the element from the
rendered layout. "visibility: hidden" only makes it transparent while
keeping its original box properties (width, height, etc).
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""" CW - nevow/twisted client
"""