Add ability to serve static files.

Added a configuration key, `service.staticpath`, to allow the serving
of files from the provided path. By default, the path is empty and
Vikunja's existing behaviour is unchanged.

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

Being able to serve static files allows the api service to also serve
the frontend content. This adds 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.

Future improvements could embed a copy of the frontend files via the
fs.FS and embed package. Alternatively a reverse proxy could be offered
using http/httputil's ReverseProxy; this would allow another server to
maintain a default set of frontend files without embedding.

CHANGELOG.md

@@ -7,6 +7,10 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 All releases can be found on https://code.vikunja.io/api/releases.
 
+## [Unreleased]
+
+* Add static file support for simplified deployment 
+
 ## [0.18.1] - 2021-09-08
 
 ### Fixed

pkg/config/config.go

@@ -47,6 +47,7 @@ const (
 	ServiceFrontendurl     Key = `service.frontendurl`
 	ServiceEnableCaldav    Key = `service.enablecaldav`
 	ServiceRootpath        Key = `service.rootpath`
+	ServiceStaticpath      Key = `service.staticpath`
 	ServiceMaxItemsPerPage Key = `service.maxitemsperpage`
 	// Deprecated: Use metrics.enabled
 	ServiceEnableMetrics         Key = `service.enablemetrics`
@@ -274,6 +275,7 @@ func InitDefaultConfig() {
 	ServiceEnableCaldav.setDefault(true)
 
 	ServiceRootpath.setDefault(getBinaryDirLocation())
+	ServiceStaticpath.setDefault("")
 	ServiceMaxItemsPerPage.setDefault(50)
 	ServiceEnableMetrics.setDefault(false)
 	ServiceMotd.setDefault("")

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{