Added "Thanks for posts" extension

[Barbie]

This extension is a conversion of Thanks for posts MOD for phpBB 3.0 by Палыч.
Updates from MOD to extension should keep the thanks data.
General functionality:

• adds Thanks button to the posts at the end of the post buttons row
• allows thanking and removing thank for a post
• adds a feature to display posts/topics/forums rating based on the count of post thanks for them (thankslist) (configurable)
• adds a controller to display a list of users who thanked others and who got thanks for their posts
• adds a controller to display a list of top thanked posts/topics/forums (toplist) (configurable)
• adds special group permissions to control viewing of toplist and thankslist
• adds special permissions to control user abilities to thank for posts (local, forum-based) and to clean thanks list (global)
• adds "standard" permission sets to the predefined roles
• adds notifications for the thanking and thanks removing events (including board and email notifications, configurable in UCP)
• if rating displaying for all of posts/topics/forums is disabled, the toplist is unavailable