Created the new templates subfolder structure and adjusted the autoload creator

TODO:

 * Adjust the template editor
 * Check the `getTemplateGroup()` method
 * Check all template drop-down menus
Better front end preview member selection

The front end preview member selection is now a text field with autocompletion.
It uses the HTML5 datalist feature.
Dynamically fill the data list via Ajax
Detect whether the browser supports data lists
Fix a typo in the "bad request" error message
Adjust the form elements of the preview nicely to the given width
Add a log entry before merging the feature branch
Merge branch 'feature/fe-preview-member-autocomplete' into develop
Fix two JavaScript errors in the front end preview bar script
Revert: Show the correct date/time format when validating front end forms (see #5238)
Merge branch 'hotfix/3.0.4' into develop
Add the missing exception stack trace in the log files
Handle invalid dates in the user modules
Do not use the class `hide` to hide synchronization results
Adjust the template editor to the new file structure
Merge branch 'feature/templates-subfolders' into develop
Fix the login screen text field width
Better theme store link integration
More decent front end preview pane