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
```
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
