Added loading spinner for dashboard and sign-up views

# Changes #

### *NOTE*: Anywhere you want to use the loading spinner animation, simply call *startLoadingAnimation()* to bring up the animation and *stopLoadingAnimation()* to kill it.

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### images/ ###
- Deleted a couple of images that we didn't need anymore.
- Added my loading spinner gif animation - **loading-shield.gif**

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### index.html ###
- Added the loading spinner container, image, and focus background.

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### dashboard-view.js ###
- Added loading spinner on initialize.
- Kill the loading spinner when the groups have successfully loaded on the page.

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### main.js ###
- Added functions to start and stop the loading spinner animation.

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### sign-up-view.js ###
- Added loading spinner on the click event of the sign up button.
- Kill the loading spinner when the user has been successfully created.

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### main.scss ###
- Added classes for the loading spinner container, image, and focus background.
- Also refactored a bunch of stuff because it was driving me crazy.