[vseth-components] Category edit page improvements

On the category improvement page there are a few things that I would like to see improved:

• The meta categories do not autocomplete anymore. This makes it way more error prone to set the meta category as you don't know exactly how the existing meta category is spelled.
• The keyboard shortcut for adding admins/experts does not work anymore. One used to be able to add them by pressing enter and the input field would autofocus again afterwards. This allows to quickly add multiple users.
• There are missing margins on top of the subtitles (e.g. admins, experts).
• The 'has payments' box looks really weird because the label is on the left side while the checkbox is in the middle.