Use pseudo elements to style changelist-separators on Contributions

Use pseudo elements to style changelist-separators on Contributions

Currently, these separators are unnecessarily hardcoded in HTML.
This is problematic, as it limits the extent to which skins such
as Minerva and Timeless can skin these pages.

If the pseudo selector is not available (e.g. legacy browsers)
this degrades nicely to whitespace and the page is still useable.

Right now, these changes are scoped to pages uses the ContribsPager
(used by Special:Contributions)
but we can adopt them in other pages once we have this approach set in
stone (CSS)

Bug: T205581
Change-Id: Ia2c485e6058ebf4282ed2f7eb7b046b2deb03f57