ext.echo.special: Cross-skin adjustments

Out of the default MW skins these only seem to affect CologneBlue.

* Reset padding and list-style-image on ul#mw-echo-special-container
  ourselves, do not rely on the skin doing it
* Use transparent background on .mw-echo-notification (and
  semi-transparent black on hover) instead of solid white and
  light grey to accomodate colored skin backgrounds

Change-Id: I2c178627e4dbe889c4958afc41e4969aaa45a717
Merge "ext.echo.special: Cross-skin adjustments"