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
Add "Categories=Network;InstantMessaging;" to ensure that Mattermost is listed under the appropriate submenu of the application starter. Tested with KDE/Plasma 5.45 and KMenu.
