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

CHANGELOG.md

@@ -13,6 +13,8 @@ Release date: TBD
 ### Improvements
 
 #### Linux
+ - Use SVG icon for Linux application menus in place of PNG icon
+ [#815](https://github.com/mattermost/desktop/pull/815)
  - Updated categories in order to be listed under the appropriate submenu of the application starter.
  [#816](https://github.com/mattermost/desktop/pull/816)
  [#818](https://github.com/mattermost/desktop/pull/818)

electron-builder.json

@@ -38,7 +38,7 @@
         "from": "resources/linux",
         "filter": [
           "create_desktop_file.sh",
-          "icon.png",
+          "icon.svg",
           "README.md"
         ]
       }

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