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

**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