DAR-320: fix some of the defects caused by DAR-264

https://wikia-inc.atlassian.net/browse/DAR-320

Fixes:
* Not being able to click on anything in the right rail due to z-index/position relative issues.
* Text running outside of the wiki activity module.
* Search input area being too wide.

Also contains some preliminary style refactoring for the RelatedVideos module. Since this is being merged into the responsive-layout branch, it won't need to be QA'd for now.

Best viewed without whitespace in the diff: https://github.com/Wikia/app/pull/908/files?w=1