Add NetworkPolicies for Mattermost

See package MR: https://repo1.dso.mil/platform-one/big-bang/apps/collaboration-tools/mattermost/-/merge_requests/32

Also note this isn't on main which is weird, but is due to the Minio stuff: https://repo1.dso.mil/platform-one/big-bang/apps/collaboration-tools/mattermost/-/merge_requests/32#note_313231

Closes #448