Netbox-contract

* Add bulk update capability for contract assignement
* [63](https://github.com/mlebreuil/netbox-contract/issues/63) Correct an API issue on the invoice object.
* [64](https://github.com/mlebreuil/netbox-contract/issues/64) Add hierarchy to contract; New parent field created.
* [65](https://github.com/mlebreuil/netbox-contract/issues/65) Add end date to contact import form.
* Removed the possibility of add or modify circuits to contracts. The field becomes read only and will be removed in next major release.
* Make accounting dimensions optional.

* [60](https://github.com/mlebreuil/netbox-contract/issues/60) Update contract quick search to also filter on fields "External reference" and "Comments".
* [49](https://github.com/mlebreuil/netbox-contract/issues/49) Manage permissions.

Add support for Netbox 3.5 which become the minimum version supported to accomodate the removal of NetBoxModelCSVForm class (replaced by NetBoxModelImportForm) .

Add support contract assignement panel to devices.

Add a new contract asignement model to allow the assignement of contract not only to Circuits. The support for the direct Contract to Circuit relation will be removed in version 2.1.0 . In Order to migrate existing relations contract_migration.py script is provided and can be run from the django shell.