Replace uses of image_tag() with CSS (#21256).

Replace uses of image_tag() with CSS (#21256).

* Replace image_tag() with CSS classes
* Create some CSS classes where necessary
* Change "check all" positioned on the right in "Trackers -> Summary" and
  "Roles -> Permissions report" instead of left like in "Worfklow"
* Add missing title attribute in a few places
* Makes some lines shorter while at it

Patch by Daniel Ritz.

git-svn-id: http://svn.redmine.org/redmine/trunk@15068 e93f8b46-1217-0410-a6f0-8f06a7374b81