Enhancements to the Categories admin page

[John Stray]

• Enable the ability to assign posts to multiple categories. -- No. Use tags instead. - #12 moved here.
• Category names should be modifiable. Add 'edit' icon with field to update name. - #9 moved here.
• Add column that shows the number of posts in the category.
• Post quantity will link back to the post management page with a filter applied to show only posts from the clicked category.
• Filtering or sorting of Categories by name or popularity (# of posts). - #10 moved here.
• New category action icons using FontAwesome.
• Create class method updateCategory() - This will remove the old category, create the new then sort through all the posts and update the category name in each post as required.