Create categories for inventory

[CJGutz]

Wait for #727

Is your feature request related to a problem? Please describe.

Should be possible to structure items by category. On the inventory page it should be possible to filter by this category.
It is expected to have around 15 categories.

Describe the solution you'd like
Create a new model called ItemCategory inside the inventory app in its own file. It should have a name and a description. The item model can then have a foreign key to this model.
There should be a selection on the inventory list to filter by a category .

When a category is selected, the inventory list should display the description of the category if there exists one.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
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