* Add ability to specify an existing Secret (#18139)

Closes #18139

* Allow using secrets with external postgres

* Upgrade CronJob to batch/v1

* Allow using redis.auth.existingSecret

* Helmignore mastodon-*.tgz for easy local development

* Upgrade helm dependencies

* Upgrade postgresql to 11

* Allow putting SMTP password into a secret

* Add optional login to SMTP secret

This to allow setting LOGIN either in values.yaml or
in the secret.

* Switch to bitnami charts full archive

This prevents older versions from disappearing, see
https://github.com/bitnami/charts/issues/10539 for
full context.

Co-authored-by: Ted Tramonte <ted.tramonte@gmail.com>

* 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