An admin can create new categories and view all categories on index page

@SherrySeibel @cgruppioni - please take a look and let me know what you think. I think at this point we all now have a category table.
1) Edit this migration so it looks like it did before (and rename it to its old name). When you squash later it'll look like you didn't touch this file.
2) Generate a new migration that adds the uniqueness constraint.