Add categories route with new, create, index
Create form for a new post category
*Modify create action in categories controller

*Create category params
*Create category index
*Make location params in location controller private
Create categories

*Create categories table
*Create model and controller
*create partial and render all categories
Create unique constraint on category name

*New migration to add constraint on existing column
Create table to uniquely identify category/locations
Add post/category relationship to models

*Add link to create new category to posts view
Create option to add category to post
Redirect to posts after creating new category
Redirect to posts page after create new category

*Remove category index page
Only display create categories link if admin
Delete category partial

*No need to have category partial because index page no longer needed.
*Instead, available categories show up in the collection select drop
down
Change table and model name to post categories

*Clarifies relationship
*Easier to read and understand
Modify categories table to have unique name
Nest form contents under signed_in?

*This will hide form contents from anyone not signed in
Change collection select to collection checkboxes
Modify models to correct table name PostCategory
Change Post Category model to validate presense
Collection check boxes post correctly

*Added user has many posts to user model
*Added validation for category_id in post_category model
Remove empty array from post params

*Empty array was not needed because we require that a post be assigned
to a category.