chore: add Made for UDS badge to Mattermost (#90)

## Description

This adds the Made for UDS badge to Mattermost given it meets the
following:

## Integrations

### Istio

- [X] - **Must** define any external interfaces under the `expose` key.
- [X] - **Must** deploy and operate successfully with Istio injection
enabled in the namespace.
- [X] - **Should** avoid workarounds such as disabling strict mTLS peer
authentication.

### Network Policies

- [X] - **Must** define network policies under the `allow` key as
required.
- [X] - **Should** minimize network policies to specific selectors
needed for Ingress/Egress traffic.
- [X] - **May** template network policy keys to provide flexibility for
delivery customers to configure.

### Keycloak

- [X] - **Must** use and create a Keycloak client through the `sso` key
if the application provides a user login.
- [ ] - **Should** consider security options during implementation to
provide the most secure default possible (i.e. SAML w/SCIM vs OIDC).
- [X] - **Should** name the client `<App> Login` (i.e. `Mattermost
Login`) to provide login UX consistency.
- [X] - **Should** clearly mark the client id with the group and app
name `uds-<group>-<application>` (i.e. `uds-swf-mattermost`) to provide
consistency in the Keycloak UI.
- [X] - **May** end any generated secrets with `-sso` to easily locate
them when querying the cluster.
- [X] - **May** template Keycloak fields to provide flexibility for
delivery customers to configure.

### Prometheus

- [X] - **Must** implement monitors for each application metrics
endpoint using it's built-in chart monitors, the `Package` CR `monitor`
key, or manual monitors in the config chart.

## Exemptions

- [X] - **Must** minimize the scope and number of the exemptions to only
what is absolutely required by the application
- [X] - **Must** have documented rationale for any exemptions present

## Structure

- [X] - **Should** expose all configuration (`uds.dev` CRs, additional
`Secrets`/`ConfigMaps`, etc) through a Helm chart (ideally in a `chart`
or `charts` directory).
- [X] - **Should** limit the use of Zarf variable templates and
prioritize configuring packages via Helm value overrides
- [X] - **Should** implement or allow for multiple flavors (ideally with
common definitions in a `common` directory)

## Testing

- [X] - **Must** implement Journey Testing to cover the basic user flows
and features of the application, especially where an application
interacts with an external service / interface.
- [X] - **Must** implement Upgrade Testing to ensure that the current
development package works when deployed over the previously released
one.
- [X] - **Should** lint their configurations with appropriate tooling
such as [`yamllint`](https://github.com/adrienverge/yamllint) and [`zarf
dev lint`](https://docs.zarf.dev/commands/zarf_dev_lint/).

## Maintenance

- [X] - **Must** have a dependency management bot (such as renovate)
configured to open PRs to update core package and support dependencies.
- [X] - **Must** release its package to the
`ghcr.io/defenseunicorns/packages/<group>` namespace as the
application's name (i.e.
`ghcr.io/defenseunicorns/packages/uds/mattermost`)

## General

- [X] - **Must** be capable of operating within an internet-disconnected
(air-gapped) environment
- [X] - **Must** be maintained by a resourced team that is explicitly
defined as maintaining the project (i.e. in `CODEOWNERS`)

## Related Issue

Fixes #N/A

## Type of change

- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [X] Other (security config, docs update, etc)

## Checklist before merging

- [X] Test, docs, adr added or updated as needed
- [X] [Contributor Guide
Steps](https://github.com/defenseunicorns/uds-package-mattermost/blob/main/CONTRIBUTING.md#developer-workflow)
followed