bootstrap 5 template function complete

current status:
- got expiration and format selections to work
- fixed modals (password, QR-code, etc.)
- replaced glyphicons with Bootstrap icons (needs CSP relaxation to work)
- tested the different settings and combinations
- got editor tabs to change active status

to be done:
- add "Dark Mode" to translation strings
- figure out how to change prettify theme when dark mode gets selected
- check tab alignment in HTML source

lib/Persistence/ServerSalt.php

@@ -56,7 +56,7 @@ class ServerSalt extends AbstractPersistence
      */
     public static function get()
     {
-        if (strlen(self::$_salt)) {
+        if (!empty(self::$_salt)) {
             return self::$_salt;
         }