Update Button component to match current design

[edmundito]

The current Button component does not match the current design:
https://www.figma.com/file/gMNRYVJav25pdsoxAyW7Qp/00_01_WEB-APP-LIBRARIES?node-id=2%3A395

• Review the design and finalize with our designer, and implement the new button changes.
• Update component to use SCSS over styled components.
• Update component to support icon and icon side properties instead of being set by the children by each button implementation
• Add loading state and deprecate LoadingButton component
• Adjust all pages to make sure updated buttons look good
• Break down changes into separate PRs where possible

[octavia-squidington-iii]

cc @airbytehq/frontend

[timroes]

As part of this I'd suggest we could also cleanup the button component for two things:

• Merge LoadingButton into Button and just have it a property of the component. (Just noticed, this is already tracked via Make <Button /> component consistent #12705)
• Give a simple property option to add an icon onto the button (currently e.g,. we just use + as a character in the string instead of an icon).