Clarify the suffices and prefixes of setting.AppSubURL and setting.AppURL (#12999)

Also removes some unnecessary uses of fmt.Sprintf and adds documentation
strings

Signed-off-by: Andrew Thornton <art27@cantab.net>

routers/utils/utils_test.go

@@ -35,7 +35,7 @@ func TestIsValidSlackChannel(t *testing.T) {
 }
 
 func TestIsExternalURL(t *testing.T) {
-	setting.AppURL = "https://try.gitea.io"
+	setting.AppURL = "https://try.gitea.io/"
 	type test struct {
 		Expected bool
 		RawURL   string