Steps to reproduce:

1. Go to Monitoring->Hosts/Templates->Items
2. Click on existing item without custom intervals to edit it

Result:
Editing form has no Custom intervals section, which is probably good for decluttering, but denies user from adding ones.

Expected:
Custom intervals should be present from the start like in item creation form or some kind of element next to Update interval needs to be introduced to allow adding Custom intervals to an item not having them.