4397 - Add Request units to item new/edit

[sean-dickinson]

Resolves #4397

Description

• Adds the ability to assign request units to an item when creating and editing the item
• Adds the custom request units column on the items list, only for organizations that have custom units defined
• Currently if you edit an item we are clearing out the list of units and the recreating the join records from the updated selection. However there is the potential for this to make the de-normalization on the ItemUnit model less useful. If we need to keep units around that the organization has deleted or changed, we may need to reconsider this approach as I'm currently relying on the unit id's and just copying over the name based on the ids that are selected from the form.
• I maybe could have considered using accepts_nested_attributes for, but given the "syncing" that I needed to do I decided to use a more explicit path initially.

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• I've added tests for this functionality in the controller and request files and have tested it locally to ensure it is working correctly. Note that everything is behind the feature flag :enable_packs

Screenshots

[cielf]

Passes initial light testing, fwiw.

[awwaiid]

Some minor changes!

[awwaiid]

@sean-dickinson real fail over in the system specs (log)

[awwaiid]

Great! Nice and safe behind the Flipper and looks good!