Draft: Expand inventory item groups

[matejv]

fixes #226

Inventory item groups can now contain any asset. You can add specific assets to a group or specify hardware types and any assets matching type will automatically belong to the group.

Groups can be nested. Any assets belonging to child group will be counted in parent groups as well.

This allows for organization structure of inventory item groups like:

- Networking team
  |- Network switches
  |- Network routers
  |- Pluggable modules
- Servers team
  |- Servers
  |- Storage
  |- Hard disks
...

TODO:

• fix tests
• fix migration deps after merging #231 & #236
• rename inventory item groups to asset groups?

[BartZimmo]

HI,

Since version 4.3 netbox announced to deprecate Inventory Items. Not sure if this PR is useful?

Deprecation Warning

Beginning in NetBox v4.3, the use of inventory items has been deprecated. They are planned for removal in a future NetBox release. Users are strongly encouraged to begin using modules and module types in place of inventory items. Modules provide enhanced functionality and can be configured with user-defined attributes.

[matejv]

closing this in favor of "asset groups" feature as described in #226