Created admin interface to create new category

* Dashboard admin view uses _category partial
* Added validations for location and category models
* Deleted duplicate sign-in/sign-out links in layout
* Modified category index view to delete create category form section
We want to render all of the categories for each location.  The actual posts will be linked to both a category and a location, but all of the categories should apply to all of the location (e.g., when I create a new category "Housing", the location show page will capture the newly update list of categories by pulling in the category partial.