This fixes#917, where crawls added to a collection via the workflow
autoAddCollections were not successfully represented in the crawl
and page count stats in the collection after completing.
* Track collections in Crawl rather than crawls in Collection
* Add delete collection API endpoint and tests
* Precompute collection crawlCount, pageCount, and tags and add them to
GET collection responses
* Add modified field to Collection
* Update collection replay.json method
* Make add and remove crawls accept list of crawl ids
* Auto-add new workflow crawls to collections when they successfully
complete via CrawlConfig.autoAddCollections field
* Move long-running post-crawl operator tasks into asyncio task
* Make CrawlConfig.autoAddCollections updatable via /update API endpoint
