Change default Secret name to not rely on Bitnami

[jeremiahlee]

Replace common.names.fullname in the default Secret name template with mastodon.fullname

This removes all Bitnami dependencies for installs not using the Bitnami PostgreSQL, Redis, and Elasticsearch. Those charts populate the common.names.fullname template value. Installs not using these charts will receive an error. Instead, use mastodon.fullname like in secrets.yaml.

Fix mastodon/chart#60 Fix mastodon/chart#65

[jeremiahlee]

I suggest a patch version bump in the Chart.yaml version when merging if not combined with other changes.