Use `organization_url` to on-board organizations

[eloquence]

We recently added support for tracking organization URLs to the SecureDrop directory, and populated it with their current URLs. See freedomofpress/securedrop.org#675 for background. This allows us to onboard organizations using their main site URL, instead of the landing page URL, which is more likely to change.

We should switch sddir.py to parse the organization URL, which will likely require updating some existing records.

[conorsch]

This would indeed be a lot saner. It's a simple change, but right now two orgs are missing the required organization_url field:

curl -s https://securedrop.org/api/v1/directory/ | jq '.[] | select(.organization_url == "").title' -r
HuffPost
Forbidden Stories

Simple enough to add those and proceed, which I'll do after knocking out the outstanding onion name requests (#80 & #81). We should consider setting those fields as required on the model, but for now we'll just consider them required for onboarding to onion names.

[zenmonkeykstop]

There are now 3 orgs missing the organization.url field (add the Markup to the list)