Make it more clear that the source code is available

[matthewfeickert]

8c7af43 adds a "Download" link the header which links to this GitHub repository (https://github.com/UW-Madison-DSI/HFExplorer). Most people aren't going to associate "Download" with "here is the source code", so I'd suggest replacing

HFExplorer/config/defaults.json

Lines 41 to 45 in 8c7af43

	"download": {
	"text": "Download",
	"icon": "fa fa-download",
	"href": "https://github.com/UW-Madison-DSI/HFExplorer"
	}

with

 "source": { 
 	"text": "GitHub", 
 	"icon": "fa-brands fa-github",
 	"href": "https://github.com/UW-Madison-DSI/HFExplorer" 
 }