new icon: Mattermost (original, original-wordmark)

[dariakiseleva]

Double check these details before you open a PR

• PR does not match another non-stale PR currently opened
• PR name matches the format new icon: Icon name (versions separated by comma). More details here
• PR's base is the develop branch.
• Your icons are inside a folder as seen here
• SVG matches the standards laid out here
• A new object is added in the devicon.json file as seen here

This PR closes #868

Link to prove your SVG is correct and up-to-date.

https://handbook.mattermost.com/operations/operations/company-processes/publishing/publishing-guidelines/brand-and-visual-design-guidelines

[Snailedlt]

@devicons/devicon__reviewers This icon needs to be discussed before we can merge it. Please take a look at the linked issue: #868

[lunatic-fox]

@devicons/devicon__reviewers This icon needs to be discussed before we can merge it. Please take a look at the linked issue: #868

It seems quite more related to technology and development than Slack, so I think it's a valid icon.

[github-actions]

Hi!

I'm the check-bot and we have some issues with your PR:

devicon.json is not sorted correctly.
Please make sure that your icon is added in the `devicon.json` file at the correct alphabetic position
as seen here: https://github.com/devicons/devicon/wiki/Updating-%60devicon.json%60

Check our CONTRIBUTING guide for more details regarding these errors.

Please address these issues. When you update this PR, I will check your SVGs again.

Thanks for your help,
SVG-Checker Bot 😄

[github-actions]

Hi!

I'm the check-bot and we have some issues with your PR:

devicon.json is not sorted correctly.
Please make sure that your icon is added in the `devicon.json` file at the correct alphabetic position
as seen here: https://github.com/devicons/devicon/wiki/Updating-%60devicon.json%60

Check our CONTRIBUTING guide for more details regarding these errors.

Please address these issues. When you update this PR, I will check your SVGs again.

Thanks for your help,
SVG-Checker Bot 😄

[canaleal]

All svgs good. No weird artifacts and they have all been optimized.