Create/Helmify MM Instance

In the Mattermost repo create a chart that makes use of the custom resource from the operator - https://repo1.dso.mil/platform-one/big-bang/apps/collaboration-tools/mattermost

TODOs:

• Create a Helm templated instance of the Mattermost custom resource
• Add subcharts for postgres and minio instances to deploy internally for pipelines
• Add imagepullsecrets and make use of IB images
• Handle templating/integration with dependencies (postgres/minio)
• Test all functionality, update README as needed