Feature/phoenix live dashboard

[johnshaughnessy]

Add phoenix live dashboard. See also Hubs-Foundation/hubs#3769

[johnshaughnessy]

TODO:

• Add prod configuration for signing_salt
• Make sure that the way we are using encrypted cookies is sound from a security perspective. See Add cookie-based authentication hubs#3769
• I need to alter the CSP so that /telemetry/metrics, /telemetry/ret_repo_info, and /telemetry/request_logger work correctly. I temporarily removed the restrictions for script-src just to get things working on this branch.
• Does RequestLogger add any security concerns? (does it capture private request info that it shouldn't? e.g. query string parameters)
• Verify that the ecto update is safe. It appears to be safe to update ecto to 3.4.0 because the error that we were seeing has been downgraded to a warning. We should test this in production, though.
  Query returns incorrect records in associations when the foreign key is omitted from select elixir-ecto/ecto#3246 (comment)
  https://github.com/elixir-ecto/ecto/blob/master/CHANGELOG.md#v356-2021-01-20
  https://github.com/elixir-ecto/ecto_sql/blob/master/CHANGELOG.md#v354-2020-01-20