Support for external register numbers

[jaakkocsc]

When user submits an application, a register number should be issued for the application. The number is only issued at submission, not for drafts — i.e. when status changes to "Applied". Register number is generated by an external service. After register number has been generated only that register number should be presented for users/handlers/etc. instead of REMS internal id.

• support for attaching external-id to an application later (via a new event?) (in "Assign external id" (diaarinumero) command #1858)
• show internal id for applications that don't yet have an external id (in "Assign external id" (diaarinumero) command #1858)
• @jaakkocsc to find out specification for the external service (in rems-deploy repository, doc folder)
• make external service configurable

Using Tweb service, acknowledge vireilletulo for the application.

[jaakkocsc]

External service's (Tweb) specification is in the private repo.

[opqdonut]

Let's do #1858 first

[opqdonut]

Let's do something like this:

• implement a middleman that REMS pings with new applications (see Event notifications #2095)
• the middleman fetches a new register number from the external service
• ... and issues a "assing external id" ("Assign external id" (diaarinumero) command #1858) command to REMS

[opqdonut]

see also cscfi/rems-deploy#129

[jaakkocsc]

Demoed with rhp in testing environment.