diffs: compare overhaul.

- made compare and commit range pages more consistent with other commit diff pages
- removed mergerly, closes #4665. Old diff2way is replaced by new diffs with
  side-by-side mode.
- cleanup button behaviour on compare page. Added help text and
  generally improved UX
- switched file-diffs to compare page with file filter. Part of #4000
- added collapse/expand commits buttons in compare views.

rhodecode/templates/debug_style/code-block.html

@@ -9,11 +9,9 @@
 </%def>
 
 <%def name="js_extra()">
-<script type="text/javascript" src="${h.asset('js/mergerly.js', ver=c.rhodecode_version_hash)}"></script>
 </%def>
 
 <%def name="css_extra()">
-<link rel="stylesheet" type="text/css" href="${h.asset('css/mergerly.css', ver=c.rhodecode_version_hash)}"/>
 </%def>