chore(deps): upgraded all deps and resolved breaking changes

[HoukasaurusRex]

Preserved complete feature parity as far as I could while upgrading all dependencies to latest version (2 years of upgrades!)

I made sure not to add or remove anything not necessary to resolving breaking changes with dependencies like emotion/core -> emotion/react or theme-ui components.

That being said, there are two notable differences in layout/design:

1. Adjusting x coordinate of seektimestamp to 20px to the right of cursor from window instead of relative to seekbar.
   https://github.com/HoukasaurusRex/gatsby-theme-simplecast/blob/f8c7014b29d829fbd3eafc06561b8d873e2b5511/gatsby-theme-simplecast/src/components/player.js#L151-L154
2. Changing navigation links to fixed position scrollable lists instead of contributing to overall page height (page can get quite big with over a few dozen episodes)
   https://github.com/HoukasaurusRex/gatsby-theme-simplecast/blob/f8c7014b29d829fbd3eafc06561b8d873e2b5511/gatsby-theme-simplecast/src/gatsby-plugin-theme-ui/index.js#L106-L107

This was tested extensively against the demo content so let me know if there are any additional issues

[vojtaholik]

wow, I really appreciate this effort! I'll check it out later this week and hopefully merge it, thank you!

[HoukasaurusRex]

No thank you for the awesome theme! Been looking ages for something decent