Pull request: * all: move internal Go packages to internal/

Merge in DNS/adguard-home from 2234-move-to-internal to master

Squashed commit of the following:

commit d26a288cabeac86f9483fab307677b1027c78524
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Oct 30 12:44:18 2020 +0300

    * all: move internal Go packages to internal/

    Closes #2234.

internal/home/i18n.go

@@ -0,0 +1,94 @@
+package home
+
+import (
+	"fmt"
+	"io/ioutil"
+	"net/http"
+	"strings"
+
+	"github.com/AdguardTeam/golibs/log"
+)
+
+// --------------------
+// internationalization
+// --------------------
+var allowedLanguages = map[string]bool{
+	"be":    true,
+	"bg":    true,
+	"cs":    true,
+	"da":    true,
+	"de":    true,
+	"en":    true,
+	"es":    true,
+	"fa":    true,
+	"fr":    true,
+	"hr":    true,
+	"hu":    true,
+	"id":    true,
+	"it":    true,
+	"ja":    true,
+	"ko":    true,
+	"nl":    true,
+	"no":    true,
+	"pl":    true,
+	"pt-br": true,
+	"pt-pt": true,
+	"ro":    true,
+	"ru":    true,
+	"si-lk": true,
+	"sk":    true,
+	"sl":    true,
+	"sr-cs": true,
+	"sv":    true,
+	"th":    true,
+	"tr":    true,
+	"vi":    true,
+	"zh-cn": true,
+	"zh-hk": true,
+	"zh-tw": true,
+}
+
+func isLanguageAllowed(language string) bool {
+	l := strings.ToLower(language)
+	return allowedLanguages[l]
+}
+
+func handleI18nCurrentLanguage(w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("Content-Type", "text/plain")
+	log.Printf("config.Language is %s", config.Language)
+	_, err := fmt.Fprintf(w, "%s\n", config.Language)
+	if err != nil {
+		errorText := fmt.Sprintf("Unable to write response json: %s", err)
+		log.Println(errorText)
+		http.Error(w, errorText, http.StatusInternalServerError)
+		return
+	}
+}
+
+func handleI18nChangeLanguage(w http.ResponseWriter, r *http.Request) {
+	body, err := ioutil.ReadAll(r.Body)
+	if err != nil {
+		errorText := fmt.Sprintf("failed to read request body: %s", err)
+		log.Println(errorText)
+		http.Error(w, errorText, http.StatusBadRequest)
+		return
+	}
+
+	language := strings.TrimSpace(string(body))
+	if language == "" {
+		errorText := fmt.Sprintf("empty language specified")
+		log.Println(errorText)
+		http.Error(w, errorText, http.StatusBadRequest)
+		return
+	}
+	if !isLanguageAllowed(language) {
+		errorText := fmt.Sprintf("unknown language specified: %s", language)
+		log.Println(errorText)
+		http.Error(w, errorText, http.StatusBadRequest)
+		return
+	}
+
+	config.Language = language
+	onConfigModified()
+	returnOK(w)
+}