Update shell extension tasks to check for site-specific file

[zenmonkeykstop]

Status

Ready for review

Description of Changes

Fixes #6903.

During installation of the Tails shell extension:

• Ansible checks for existence of site-specific before attempting to load variables from it
• The extension JS script gets sensible defaults if site-specific is not present

Testing

• Follow docs for creation of a Journalist workstation and verify that it succeeds, with no SSH options in the shell extension
• Follow docs for creation of an Admin Workstation and verify that it succeeds, with SSH options in the shell extension.

Deployment

• Will be deployed and set up with next release
• a docs workaround may be necessary in the interim for fresh installs.

Checklist

If you made non-trivial code changes:

• I have written a test plan and validated it for this PR

Choose one of the following:

• I have opened a PR in the docs repo for these changes, or will do so later
• I would appreciate help with the documentation
• These changes do not require documentation

[nathandyer]

All the changes here LGTM

Testing

• Follow docs for creation of a Journalist workstation and verify that it succeeds, with no SSH options in the shell extension
  • Brand-new USB stick with Tails freshly installed
  • ./securedrop-admin tailsconfig succeeds without error
  • SecureDrop Menu loads on Tor connection after next reboot
  • SSH options do not appear
• Follow docs for creation of an Admin Workstation and verify that it succeeds, with SSH options in the shell extension.
  • ./securedrop-admin tailsconfig succeeded without error
  • SecureDrop Menu loads on Tor connection after next reboot
  • SSH options in the shell extension on reboot
  • SSH options functional

Not a maintainer so I can't merge, but this should be ready for a stamp.

@zenmonkeykstop please let me know what (if any) changes we need to make to the docs prior to this fix making it to an official release, and I'll get a docs PR filed. Thanks!