The cronjob tries to get key from `mastodon` secret instead of
`mastodon-postgresql` - so the cronjob fails with this error:

Error: couldn't find key postgresql-password in Secret [NS]/mastodon

Another solution is to save the postgres password in mastodon secret,
but that means that the password is placed in two places.

Postgresql use <fullname>-postgresql name as secret name.

* add Helm chart

known issues/future work:

- SSO is unsupported

- S3/Minio/GCS is unsupported

- Swift is unsupported

- WEB_DOMAIN is unsupported

- Tor is unsupported

* helm: clarify how LOCAL_DOMAIN is set

* helm: add chart description

* helm: make DB_POOL and Sidekiq concurrency configurable

* helm: only enforce pod affinity when using ReadWriteOnce

* helm: clarify compatibility

* helm: clean up application variables

* helm: add job to create initial admin