fix(create_shell_extension): import {app,monitor}_hostname site-specific variables

[cfm]

Status

Ready for review

Description of Changes

Fixes #6847 by importing, rather than reading and then setting new global facts for, the app_hostname and monitor_hostname site-specific variables.

Testing

1. If you were involved in testing Add a GNOME Shell Extension #6712, check that Admin Workstation's /home/amnesia/.ssh/config and observe templating as shown in [2.6.0-rc1] Application Server's SSH alias is configured incorrectly #6847 (comment).
2. From this branch, securedrop-admin --force tailsconfig.
3. Check ~/.ssh/config again and confirm that both the {app,mon} aliases and the servers' actual hostnames (if different than the aliases) are presented.
4. ssh $HOSTNAME works for each aliased hostname.
5. If the hostnames aren't different, bonus points for running securedrop-admin sdconfig with junk hostnames and repeating steps (1)‒(3).

Deployment

After merge, backport this branch into release/2.6.0 and add a check for #6847 in the v2.6.0 test plan.

[zenmonkeykstop]

Checklist checks out including bonus points