Removes the following 2 redundant queries from the deletion process:
```
SELECT "tags_tagkey"."id" FROM "tags_tagkey" WHERE "tags_tagkey"."project_id" IN (1) ORDER BY "tags_tagkey"."project_id" ASC, "tags_tagkey"."id" ASC LIMIT 498
UPDATE "projects_project" SET "stored_event_count" = ("projects_project"."stored_event_count" - 1) WHERE "projects_project"."id" = 1
```
Like e45c61d6f0, but for .project.
I originally thought `SET_NULL` would be a good way to "do stuff later", but
that's only so the degree that [1] updates are cheaper than deletes and [2]
2nd-order effects (further deletes in the dep-tree) are avoided.
Now that we have explicit Project-deletion (deps-first, delayed, properly batched)
the SET_NULL behavior is always a no-op (but with cost in queries).
As a result, in the test for project deletion (which has deletes for many
of the altered models), the following 12 queries are no longer done:
```
SELECT "projects_project"."id", [..many fields..] FROM "projects_project" WHERE "projects_project"."id" = 1
DELETE FROM "projects_projectmembership" WHERE "projects_projectmembership"."project_id" IN (1)
DELETE FROM "alerts_messagingserviceconfig" WHERE "alerts_messagingserviceconfig"."project_id" IN (1)
UPDATE "releases_release" SET "project_id" = NULL WHERE "releases_release"."project_id" IN (1)
UPDATE "issues_issue" SET "project_id" = NULL WHERE "issues_issue"."project_id" IN (1)
UPDATE "issues_grouping" SET "project_id" = NULL WHERE "issues_grouping"."project_id" IN (1)
UPDATE "events_event" SET "project_id" = NULL WHERE "events_event"."project_id" IN (1)
UPDATE "tags_tagkey" SET "project_id" = NULL WHERE "tags_tagkey"."project_id" IN (1)
UPDATE "tags_tagvalue" SET "project_id" = NULL WHERE "tags_tagvalue"."project_id" IN (1)
UPDATE "tags_eventtag" SET "project_id" = NULL WHERE "tags_eventtag"."project_id" IN (1)
UPDATE "tags_issuetag" SET "project_id" = NULL WHERE "tags_issuetag"."project_id" IN (1)
```
Implemented using a batch-wise dependency-scanner in delayed
(snappea) style.
* no real point-of-entry in the (regular, non-admin) UI yet.
* no hiding of Projects which are delete-in-progress from the UI
* lack of DRY
* some unnessary work (needed in the Issue-context, but not here)
is still being done.
See #50
