add initial Helm chart

[themightychris]

Add a helm chart for deploying an instance of AgencyHub to a Kubernetes namespace

LetsPlan/OurPlan and CfP's Vaultwarden extension both provide good examples of deploying a postgres statefulset alongside an application, with the following notes:

• AgencyHub doesn't need to deploy metabase like LetsPlan does, since any data in AgencyHub's database that's useful for analysis should get pulled into the warehouse by an external process.
• AgencyHub's chart should support disabling the built-in postgres instance in favor of configuring an external postgres server
• Should include a cron job for snapshotting to a bucket with restic, both examples show a similar implementation of this