Upgrade phoenix

[johnshaughnessy]

Currently we use

• mozillareality/erlang/22.0 (habitat.sh)
• mozillareality/erlang/22.0 (reticulum.io)
• core/elixir/1.8.0 (habitat.sh)
• core/elixir/1.8.0 (reticulum.io)
• phoenix 1.4.17 (2020-04-23)

I want to upgrade to phoenix.

• Versions 1.4.1 - 1.4.17 have a boat load of bug fixes edit: I thought we were on 1.4.0 until I checked our mix.lock file.
• Version 1.5.x allows us to add phoenix_live_dashboard (for ourselves AND for hubs cloud operators.)
• Version 1.6 is currently in development and requires upgrading elixir to 1.9+. There does not seem to be a compelling reason to upgrade so I recommend we stay on 1.5.x. But if we wanted to, elixir habitat packages are available.

I want to upgrade absinthe.

• 1.5 includes built in telemetry instrumentation and a bunch of bug fixes.

• 1.6 reduces memory consumption of subscriptions and has a bunch of bug fixes.

  As far as I can tell, these upgrades will not require any changes to the CloudFormation Stack template.

[johnshaughnessy]

This work is partially done in https://github.com/mozilla/reticulum/pull/456 and in https://github.com/mozilla/reticulum/pull/404