Put overhauled icon on baseline and unify code with Vector's

Putting icon on link label baseline as good as possible cross-browser
and unify code with Vector skin's code. With one improvement,
putting icon in `:before` selector in order to be able to specifically
change icon independent from link, which opens up `opacity` or state
changes.
Also cleaning up CSS by
- reducing specificity
- changing to `stylelint-disable-line` only on specific selectors
- removing initial value `transparent`

Bug: T207075
Change-Id: Ib5d446434263441eb85aa2e96a0e4dbde1034c1c
Depends-on: I92acb9851a3c0acdbc40a4a4528a91c7332c9293

resources/css/ext.uls.pt-monobook.less

@@ -1,6 +1,4 @@
-/* stylelint-disable selector-max-id */
-
-#pt-uls a.uls-trigger {
+#pt-uls .uls-trigger { // stylelint-disable-line selector-max-id
 	// Use same padding-left as #pt-userpage
 	padding-left: 20px;
 }