feat: Add partners entity

[jeppekroghitk]

Link to ticket

#8085

Description

Add a partner entity to the system, for coupling initiatives with the partners
collaborating on a project. Implemented with a multiselect and quick-add functionality.

The partner entity comes with a CRUD in the administration panel, for completing and managing existing partners. When a partner is "quick-added", only the name is filled out. The user will be nudget to fill out the remaining information about the partner via other sources (Blink or the "Dit igangværende arbejde" panel on the dashboard)

Upon deletion of a partner in the CRUD, display a prompt listing the initiatives that utilized said partner. Allow the user to delete the partner anyway, which will delete the relation on the given initiatives as well.

Screenshot of the result

N/A

Checklist

• My code is covered by test cases.
• My code passes our test (all our tests).
• My code passes our static analysis suite.
• My code passes our continuous integration process.