Adding/Removing contacts is very inconsistent

[GewoonJaap]

Describe the bug

The way of adding/removing a contact to a company/project group is very inconsistent.
On the company page you can only add/remove a contact on the show page.
On the project group pages you can add/remove a contact on both the show and edit/create page.
The design is also different on these pages.

Design on the project group create/edit page is also messy, as mentioned in #118

To Reproduce

Steps to reproduce the behavior:

1. Go to the company/project group show page
2. Open another window with the project group edit/create page
3. Open another windows with the company edit page
4. Observe

Expected behavior

The way of adding/removing a contact should be the same on all the pages

Screenshots

Additional context

Filtering contacts on the project group edit page is missing too.