http: add "Use reverse proxy host" option

added "Use reverse-proxy reported "Host" when possible" option, which
will prefer using the "Host" or "X-Forwarded-Host" values instead of
"External jfa-go URL" in the web app. To do so, app.ExternalDomain/URI
are now functions which take *gin.Context (the latter optionally). The
protocol for the request is determined from X-Forwarded-Proto(col), so
make sure your proxy includes it.

The wiki will have been updated to mention the new option.

main.go

@@ -121,7 +121,8 @@ type appContext struct {
 	host                        string
 	port                        int
 	version                     string
-	ExternalURI, ExternalDomain string
+	externalURI, externalDomain string // The latter lower-case as should be accessed through app.ExternalDomain()
+	UseProxyHost                bool
 	updater                     *Updater
 	webhooks                    *WebhookSender
 	newUpdate                   bool // Whether whatever's in update is new.