Refactors tags and adds tag requests specs

[dennyluan]

This PR refactors the tag endpoints and adds in tag request specs. 🎉

Going to try and follow the pattern here for the other resources as I think it's a little bit cleaner. Those will be separate PRs.

[cindywu]

I pulled down the branch and successfully

• created a tag
• added the tag to a post
• updated the description of a tag

Note: In this PR a description column was introduced to the Tag table, but description is not displayed in Tag view.