Update icon for Linux menus to use an SVG icon

Replace mattermost-desktop PNG icon (icon.png) with an SVG version
(icon.svg). This will improve icon appearance on Linux desktops.
Update icon references in electron-builder.json and
create_desktop_file.sh to point to the new SVG icon.
Update CHANGELOG.MD
Fixes #507

resources/linux/create_desktop_file.sh

@@ -12,7 +12,7 @@ Comment=Mattermost Desktop application for Linux
 Exec="${FULL_PATH}/mattermost-desktop"
 Terminal=false
 Type=Application
-Icon=${FULL_PATH}/icon.png
+Icon=${FULL_PATH}/icon.svg
 Categories=Network;InstantMessaging;
 EOS
 chmod +x Mattermost.desktop