[2.5] End of Support Notification Plugin

On behalf of the Update Working Group, this plugin is proposed to be included in the final 2.5 release in order to notify users of the end of support and to provide additional resources for preparing a migration to the 3.x branch.  Users who migrate to 3.x with this plugin will have it uninstalled at that time (PR for that branch forthcoming).

Committer: Please attribute @crusonweb for this pull as he was the person who wrote the plugin, myself and George just moved the code into the branch here and assisted with the update scripts.
@wilsonge as you delete the stop15.png this can be also remove here. :D

https://github.com/joomla-projects/joomla-cms/commit/954d5feb10b9b1fd8ce69c68c27a853cc4214d95