Ignore HM start requests if droplet_sha not yet known

This way, the crash logs for a newly deployed, crashed app can be viewed
immediately after cf push.

[#55409154]
Check if app has staged in ignoring HM start

[#55409154]
remove visualize_models script (and graphviz dep)

graphviz is GPL.

also the script probably doesn't work anymore.

[finishes #55272598]