Fix some problems when editing an existing meal item
The field 'amount_gramm' is always required
Remove the help text about seting the time on meal items, no longer needed
Fix some small issues, improve the display of units in drop downs
Set the correct CSS classes on dropdown menus in generic views
Don't use a hover effect on action icons on the unit overview
Make the field 'amount_gramm' a decimal (useful for weight units)
Rename field 'amount_gramm' to 'amount'