Fixed functionality of sign in and sign up buttons/links

# Changes #

### index.html ###
- Added a template we can use for displaying alert boxes.

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### home-view.js ###
- Fixed sign up button so that it takes you to sign up view when clicked and you're not logged in.
- If you're logged in and click the sign up button, it will display an alert to let you know you're already logged in.

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### user-nav-view.js ###
- When you click the sign out button, it will reroute you to the home page after signing you out.

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### main.scss ###
- Fixed the sign up button so that it centers properly on the page.
- Added styling for an alert popup.