Integrate Mattermost Operator with Big Bang Chart

Create mattermost operator as a new add on to the Big Bang chart. Ensure values mapping for any necessary pieces.

TODOs:

• Create new helmrelease, gitrepo, namespace for the operator
• Modify the chart values to add the operator
• Enable the operator in the k3d values
• Test it all and make sure it works...looking at you @karchaf