Donate link crashes program when clicked on system without xdg-open #437

Describe the bug
Lazydocker doesn't check if xdg-open is installed before attempting to run it, and crashes if it's missing.

To Reproduce
Steps to reproduce the behavior:

1. Install Lazydocker on a machine without xdg-open installed (such as a headless server)
2. Click donate link
3. Crash

Expected behaviour
Lazydocker should prompt (possibly with a link to manually copy) that it can't open the page itself.

Desktop:

• OS: Headless Arch Linux server
• Lazydocker Version 0.18.1