Make sure that every item in the UI configuration can build a shelf

[julianstirling]

Once #79 is merged the setting of what configuration options are held in the UI will be the job of gen_nimble_conf_options.py

Currently this shows everything, even though some cannot create a shelf. We should probably:

• Hide the item if there is no dimension information provided
• If the height in units is not set for the shelf, then we calculate it from the item height. (We may want to always calculate it rather than have the info in the database?)
• We also want to consider what to do for items that don't have a normal shelf such as
  • The access points that clip into the lid of a case
  • The items that as standard are made into a special top for the rack. (How do we handle if two of these are chosen?, do we make that impossible, do they also have a shelf style?)