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/Configuration.php

@@ -240,7 +240,7 @@ class Configuration
 
         // ensure the basepath ends in a slash, if one is set
         if (
-            strlen($this->_configuration['main']['basepath']) &&
+            !empty($this->_configuration['main']['basepath']) &&
             substr_compare($this->_configuration['main']['basepath'], '/', -1) !== 0
         ) {
             $this->_configuration['main']['basepath'] .= '/';