Small refactorization of avatar methods (#31391).

Small refactorization of avatar methods (#31391).

* move existing methods from ApplicationHelper to a new helper file (AvatarsHelper)
* change default avatar size from 50 to 24 because most of the avatars are using the  size 24
* class 'gravatar' is always added and all custom classes are appended
* added user name as default title for avatar images
* added two new methods: @assignee_avatar@ and @author_avatar@

Patch by Marius BALTEANU.

git-svn-id: http://svn.redmine.org/redmine/trunk@18175 e93f8b46-1217-0410-a6f0-8f06a7374b81