Add outline for default windows tray icon

[raaaste]

My Setup

• Operating System: Windows 7
• App Version: Windows client Version 2.13.1

• I have tested with the latest version
• I can simulate the issue easily

Description

The default white icon is not very convenient for users with Win 7. See screenshot. If the icon is in the tray menu and the user doesn't hover over the tray icon, there is no way to tell rocketchat is there.
screenshot

Current Behavior

The default icon is white making it difficult for windows 7 users to tell rocketchat is in the tray.

Expected Behavior

The icon should have for example a gray outline, so it's easier to spot against a white background.