Add imagePullSecrets and use the Ironbank images from registry1 for Mattermost

This is currently blocked by an issue with the operator - https://repo1.dso.mil/platform-one/big-bang/apps/collaboration-tools/mattermost-operator/-/issues/1.

The Mattermost custom resource needs to be update to make use of imagePullSecrets and use the image from Ironbank.

Definition of done:

• Mattermost resource updated to pass imagePullSecrets to the deployment
• Values updated to use the Ironbank image
• Docs, changelog, etc updated