Add support for multiple staticDirs

This commit adds support for multiple statDirs both on the global and language level.

A simple `config.toml` example:

```bash
staticDir = ["static1", "static2"]
[languages]
[languages.no]
staticDir = ["staticDir_override", "static_no"]
baseURL = "https://example.no"
languageName = "Norsk"
weight = 1
title = "På norsk"

[languages.en]
staticDir2 = "static_en"
baseURL = "https://example.com"
languageName = "English"
weight = 2
title = "In English"
```

In the above, with no theme used:

the English site will get its static files as a union of "static1", "static2" and "static_en". On file duplicates, the right-most version will win.
the Norwegian site will get its static files as a union of "staticDir_override" and "static_no".

This commit also concludes the Multihost support in #4027.

Fixes #36
Closes #4027

hugolib/config.go

@@ -14,6 +14,7 @@
 package hugolib
 
 import (
+	"errors"
 	"fmt"
 
 	"io"
@@ -88,7 +89,7 @@ func LoadConfig(fs afero.Fs, relativeSourcePath, configFilename string) (*viper.
 	return v, nil
 }
 
-func loadLanguageSettings(cfg config.Provider) error {
+func loadLanguageSettings(cfg config.Provider, oldLangs helpers.Languages) error {
 	multilingual := cfg.GetStringMap("languages")
 	var (
 		langs helpers.Languages
@@ -104,7 +105,56 @@ func loadLanguageSettings(cfg config.Provider) error {
 		}
 	}
 
+	if oldLangs != nil {
+		// When in multihost mode, the languages are mapped to a server, so
+		// some structural language changes will need a restart of the dev server.
+		// The validation below isn't complete, but should cover the most
+		// important cases.
+		var invalid bool
+		if langs.IsMultihost() != oldLangs.IsMultihost() {
+			invalid = true
+		} else {
+			if langs.IsMultihost() && len(langs) != len(oldLangs) {
+				invalid = true
+			}
+		}
+
+		if invalid {
+			return errors.New("language change needing a server restart detected")
+		}
+
+		if langs.IsMultihost() {
+			// We need to transfer any server baseURL to the new language
+			for i, ol := range oldLangs {
+				nl := langs[i]
+				nl.Set("baseURL", ol.GetString("baseURL"))
+			}
+		}
+	}
+
 	cfg.Set("languagesSorted", langs)
+	cfg.Set("multilingual", len(langs) > 1)
+
+	// The baseURL may be provided at the language level. If that is true,
+	// then every language must have a baseURL. In this case we always render
+	// to a language sub folder, which is then stripped from all the Permalink URLs etc.
+	var baseURLFromLang bool
+
+	for _, l := range langs {
+		burl := l.GetLocal("baseURL")
+		if baseURLFromLang && burl == nil {
+			return errors.New("baseURL must be set on all or none of the languages")
+		}
+
+		if burl != nil {
+			baseURLFromLang = true
+		}
+	}
+
+	if baseURLFromLang {
+		cfg.Set("defaultContentLanguageInSubdir", true)
+		cfg.Set("multihost", true)
+	}
 
 	return nil
 }
@@ -178,5 +228,5 @@ func loadDefaultSettingsFor(v *viper.Viper) error {
 	v.SetDefault("debug", false)
 	v.SetDefault("disableFastRender", false)
 
-	return loadLanguageSettings(v)
+	return loadLanguageSettings(v, nil)
 }