Added mattermost (#1712)

Closes #1552

_data/simple-icons.json

@@ -1865,6 +1865,11 @@
             "hex": "000000",
             "source": "https://matrix.org"
         },
+        {
+            "title": "Mattermost",
+            "hex": "0072C6",
+            "source": "https://www.mattermost.org/brand-guidelines/"
+        },
         {
             "title": "Matternet",
             "hex": "261C29",

icons/mattermost.svg

@@ -0,0 +1 @@
+<svg role="img" viewBox="0 0 24 24" xmlns="http://www.w3.org/2000/svg"><title>Mattermost icon</title><path d="M12.081 0C7.048-.034 2.339 3.125.637 8.153c-2.125 6.276 1.24 13.086 7.516 15.21 6.276 2.125 13.086-1.24 15.21-7.516 1.727-5.1-.172-10.552-4.311-13.557l.126 2.547c2.065 2.282 2.88 5.512 1.852 8.549-1.534 4.532-6.594 6.915-11.3 5.321-4.708-1.593-7.28-6.559-5.745-11.092 1.031-3.046 3.655-5.121 6.694-5.67l1.642-1.94C12.24.001 12.16 0 12.08 0zm3.528 1.094a.284.284 0 00-.123.024l-.004.001a.33.33 0 00-.109.071c-.145.142-.657.828-.657.828L13.6 3.4l-1.3 1.585-2.232 2.776s-1.024 1.278-.798 2.851c.226 1.574 1.396 2.34 2.304 2.648.907.307 2.302.408 3.438-.704 1.135-1.112 1.098-2.75 1.098-2.75l-.087-3.56-.07-2.05-.047-1.775s.01-.856-.02-1.057a.33.33 0 00-.035-.107l-.006-.012-.007-.011a.277.277 0 00-.229-.14z"/></svg>