Add a Delete function to the Edit Product Relationship form

[DennisClark]

Is your enhancement request related to a problem? Please describe.
Cumbersome to delete an old version of a Package or Component from a Product Inventory.

What are the benefits of the requested enhancement?
Facilitate Product Inventory management, especially when adding new versions of existing items.

Describe the solution you would like
Add a Delete function to the Edit Product Relation (Package/Component) form (a button at the bottom).

[tdruez]

The "Delete" button was not properly migrated during the Inventory tab refactoring in https://github.com/nexB/dejacode/pull/24/files

Some permissions where not available in the tab context anymore and the button was hidden.
Fixed in #134