Fix rounded corners issue on help button by destroying notices

Empty this.$element in notices when there are no elements to display
as just hiding with CSS makes other items in the group no the last
child and therefore breaks any CSS rounded corners rules.

As notices are only ever generated at load, destroying the tool
is not a problem.

Bug: T63575
Change-Id: I63a044ca63f61b976f57e34ce9537034e68a2614
Create ve.ui.MWCommandHelpDialog.css for height hack

Height differs to core due to extra commands.

Change-Id: I8162ff4db86ceeac0ca92d165b185a418f9e4fc7
Merge "Fix rounded corners issue on help button by destroying notices"
Merge "Create ve.ui.MWCommandHelpDialog.css for height hack"