Feature/refactor activity feed

-add small discussion icon for activity feed
-remove rendant proposal icon
-add button and modal for editing the motion close date(admin/author only)
-added a discussion_id to events table to reuse for activity feed on discussion page.(all entries with -----discussion_Id not null are activity.)
-removed all references to history. changed classes and styling to suit
-refactored _activity partial (previously _history), to call an _activity_item partial for each case of activity
-moved all controller event logic to models.
-changed 'delete comment ' from destroy to archive. Setting a achieved_at date on the comments
-put a condition on the comment relationship to the discussion to support the above in cases of ---'discussion.last_comment_at'
-new activity recorded for 'edit_close_date', 'edit_discussion_description' and 'edit_discussion_title'
-ajax updates of activity feed added for last two above.
-description added to new discussion mailer
-refactor activity items with polymorphism
-move icon to separate css file
-refactor position icon partial