Deprecate Refinery::Plugin#version.

AFAIK Plugin#version is not used anywhere.
Merge remote-tracking branch 'origin/remove_plugin_version'
Improved caller logic in Refinery#deprecate functionality.
Fixed line ending.

[ci skip]
Added ability to override helper.
Use hash-sign for numbered list in "8 - Extending Models" guide
Return nil if CSS array has no contents.

Fixes #2138
Merge remote-tracking branch 'kryzhovnik/adds_ability_to_override_helpers'
Minor engine template fix.

It makes no sense to show "sorting links" if there are not many of a resource.

Fixes #2142
merge upstream changes