Change the Joplin Favicon to something less wagtail

### As a dev (and also general joplin user) I want joplin to have a distinct favicon so that I can easily find Joplin in my browser tabs.

**Is your feature request related to a problem? Please describe.**
When I have joplin in a tab, but also have numerous wagtail docs tags open, it's harder for me to find which tab is Joplin.

**Describe the solution you'd like**
Joplin to have a pretty icon. Also a dream could be for staging/local to have a different icon. A love chicken with a hardhat??

**Describe alternatives you've considered**
I have considered not looking at so many wagtail docs, but that can't happen.