Add comments model and routes

This adds a comments model with a "commentable" polymorphic association.

In order to use the comment model on a different model, add the
commentable concern with `include Commentable` and apply the comments
concern for the routes.
Add comment counter cache

This adds a counter cache for comments to libraries
Add basic comment listing for single library view

This is still missing any ajax functionality
Fix style issues mentioned by houndci
Fix more houndci complaints
Fix library test
Add basic i18n for comments
Order comments by newest first
Basic comment form

Adds a very basic functional comment form.
Page still needs to be reloaded in order to show new comments.
Only show comment form if user logged in
Prompt unregistered to login for creating comments
Extract library_comments partial
Use ajax for displaying created comment

Also added basic design to the form elements
Make "No Comments" disappear after first comment
Add "show more" link for comments

This adds a "show more" link for comments.
Currently, it loads 5 comments per click.
If there are no more comments, the link is not shown anymore.
CSS improvements
Moved user slides in welcome page to the bottom.
Removed links to 'philosophy' page from footer and content pages.
Add comment deletion functionality

Users can now delete their own comments
repair heart styles