Icon Request: icon-mattermost
### Acknowledgement

- [X] I have searched through both open and closed issues and confirm that this icon has not been requested earlier.

### Icon

https://github.com/mattermost/desktop/blob/master/resources/windows/tray.svg

### Name suggestion

mattermost

### Use-case

Using this on a contact-page, list of web-based software in a company, tray-icon for waybar, …

### Additional information for brand icons

From Wikipedia:
Mattermost is an open-source, self-hostable online chat service with file sharing, search, and integrations. It is designed as an internal chat for organisations and companies, and mostly markets itself as an open-source alternative to Slack and Microsoft Teams.
It is integrated into GitLab-Omnibus, used by Wikimedia in Wikimedia Chat on Wikimedia Cloud Services as of late summer 2020 and is used in multiple German universities as an GDPR-conform online chat service.

Licence: https://github.com/mattermost/desktop/blob/master/LICENSE.txt