Support changing default landing path

This adds a new `landing` value in the [app] section of config.ini.
If non-empty, unauthenticated users on multi-user instances will be
redirected to the path given there.

This closes T574

app.go

@@ -31,6 +31,7 @@ import (
 	"github.com/gorilla/sessions"
 	"github.com/manifoldco/promptui"
 	"github.com/writeas/go-strip-markdown"
+	"github.com/writeas/impart"
 	"github.com/writeas/web-core/auth"
 	"github.com/writeas/web-core/converter"
 	"github.com/writeas/web-core/log"
@@ -90,6 +91,10 @@ func handleViewHome(app *App, w http.ResponseWriter, r *http.Request) error {
 		return handleViewPad(app, w, r)
 	}
 
+	if land := app.cfg.App.LandingPath(); land != "/" {
+		return impart.HTTPError{http.StatusFound, land}
+	}
+
 	p := struct {
 		page.StaticPage
 		Flashes []template.HTML

config/config.go

@@ -13,6 +13,7 @@ package config
 
 import (
 	"gopkg.in/ini.v1"
+	"strings"
 )
 
 const (
@@ -64,6 +65,7 @@ type (
 		Theme      string `ini:"theme"`
 		JSDisabled bool   `ini:"disable_js"`
 		WebFonts   bool   `ini:"webfonts"`
+		Landing    string `ini:"landing"`
 
 		// Users
 		SingleUser       bool `ini:"single_user"`
@@ -134,6 +136,13 @@ func (cfg *Config) IsSecureStandalone() bool {
 	return cfg.Server.Port == 443 && cfg.Server.TLSCertPath != "" && cfg.Server.TLSKeyPath != ""
 }
 
+func (ac *AppCfg) LandingPath() string {
+	if !strings.HasPrefix(ac.Landing, "/") {
+		return "/" + ac.Landing
+	}
+	return ac.Landing
+}
+
 // Load reads the given configuration file, then parses and returns it as a Config.
 func Load(fname string) (*Config, error) {
 	if fname == "" {