[v3 onion migration] securedrop-admin sdconfig should set v2_onion_services and v3_onion_services variables

[redshiftzero]

Description

We should enable the setting of two new boolean configuration variables in site-specific by securedrop-admin sdconfig: v2_onion_services and v3_onion_services.

The expected behavior here is:

• The default value of the v2_onion_services variable is set to:
  • True if there is a ths file (app-ths-source) files with a 16 char onion address in install_files/ansible-base/ - this is to prevent admins from accidentally disabling v2 onion services
  • False otherwise - we don't want new installs to be using v2 services beginning in SecureDrop 1.0.0
• The default value of the v3_onion_services variable is set to True:
  • we want new SecureDrop instances to only use v3 services beginning in SecureDrop 1.0.0 and we want existing installs to enable v3 alongside v2 so they can begin transitioning users to v3.

Note that we may need to do another iteration to improve the messaging to admins / point to some overview docs (to be written) in docs.securedrop.org explaining the migration process. This ticket is primarily just for the functionality of setting the proper defaults (the docs and messaging in sdconfig should also communicate that users using HTTPS on the source interface will need to get a new certificate including the v3 interface before advertising source users to use the v3 onion service).

Subticket of #2951