Add button mixins

I have taken the classes out of the HTML markup and used Sass mixins in the `.scss` files.

I had to *"eye"* the `Login.html`, `Register.html` and `PostDetail.html` views, since they are not set up in the app.

I need to **DOUBLE CHECK** these when merged!

I also changed all **RED** action buttons to **GREEN** to signify a positive action as opposed to negative action.