Refine analytics to track page events

Want to track (an aspirational list):

- How many visitors use copy-and-paste feature
- Most popular color schemes used
- How many times rgb vs hsl vs hexadecimal values are used.
- How many times a scheme gets (re-)generated 
- How many times visitors open the glossary modal.

## Integrate Google + React

https://github.com/react-ga/react-ga