Fix: Duplicate projects in query

[david-code]

Description

Caused by #870

When memberships are deleted, they are still included in the Django automatic joins between projects, membership lists and memberships. For example, here is the SQL generated by trying to select for related memberships from a project (made some manual edits for clarity when debugging):

SELECT p.name, ml.deleted_at AS ml_deleted, m.deleted_at AS m_deleted FROM "project_management_project" AS p
INNER JOIN "collaboration_membershiplist" AS ml
ON (p."membership_list_id" = ml."id")
INNER JOIN "collaboration_membership" AS m
ON (ml."id" = m."membership_list_id")
WHERE (m."user_id" = 'be080997-4edc-49cd-8f47-c69d2a3749e2'
AND p."deleted_at"
IS NULL) ORDER BY p."created_at" ASC;

You can see only projects are being checked if deleted_at IS NULL, when memberships and membership lists should be checked as well.

The easiest fix for now is just to add this check explicitly, like I've done. Eventually we need to figure out why this is not behaving as I expect.