Component: Back-to-top Button

Problem/Motivation

Back to top button is showing up on more and more sites but each one is built different and doesn't include animation half the time.

Back to top buttons should be consistent across sites and functionality and animation should be consistent.

1. design has requested for this to always have an animation. Style is open to whatever for default. Focus is on consistent movement
2. CSS only animation examples: https://now.tufts.edu/#top
   1. Uses scroll-behavior: smooth;
3. JS: https://populationcdev.wpengine.com/

Please decide on a final direction (CSS vs JS) and finalize a transition speed based on the examples above.