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