feat: ability to serve static files (#1174)

Added the configuration key, `service.staticpath`, to serve files from the configuration path on root (/).

Serving static files allows the api service to also serve the frontend content. This is a simple option for deploying Vikunja without needing any other servers or proxies.

Running a complete instance becomes:

    VIKUNJA_SERVICE_STATICPATH=/path/to/frontend ./vikunja

Where `/path/to/frontend` is a copy of Vikunja's frontend static files.

## Implementation

Providing a path, via the configuration or environment, adds a static file middleware to serve the path's contents from root (/).

By default, the configuration path is empty and Vikunja's existing behaviour is unchanged.

Co-authored-by: Graham Miln <graham.miln@dssw.co.uk>
Reviewed-on: https://kolaente.dev/vikunja/api/pulls/1174
Reviewed-by: konrad <k@knt.li>
Co-authored-by: grahammiln <grahammiln@noreply.kolaente.de>
Co-committed-by: grahammiln <grahammiln@noreply.kolaente.de>

pkg/routes/routes.go

@@ -203,6 +203,11 @@ func RegisterRoutes(e *echo.Echo) {
 	// healthcheck
 	e.GET("/health", HealthcheckHandler)
 
+	// static files
+	if static := config.ServiceStaticpath.GetString(); static != "" {
+		e.Use(middleware.Static(static))
+	}
+
 	// CORS_SHIT
 	if config.CorsEnable.GetBool() {
 		e.Use(middleware.CORSWithConfig(middleware.CORSConfig{