Change tab size in diffs

ZD: https://gitlab.zendesk.com/agent/tickets/15488

Description

Change the tab size in diffs for an omnibus installation. As it seems to be possible to change this in a css file (http://feedback.gitlab.com/forums/176466-deprecated-feedback-forum/suggestions/3983255-customizable-tab-width-in-diffs) there should be an option for omnibus users as well.