Update to Python 3.8 and bugfixes

[gene1wood]

• Update to use Python 3.8
  • This also produces a new artifact zip with new libraries built for 3.8
• Update build-and-upload-birch-girder doc to include docker method
  • Also remove out of date deployment methods from before we switched to using Lambda Layers
• Fix setting reaction on initial email
  • This was caused by an oddity of agithub
  • Fixes #32
• Switch reaction to sending an email from heart to rocket
  • Seems like a rocket is a better indication of sending something whereas a heart is more of an endorsement of something
• Render initial email reply into HTML from Markdown for HTML email
• Various small fixes and tweaks