[v3 onion migration] Modify securedrop-admin tailsconfig to use v3 if available

[redshiftzero]

Description

This ticket is to modify securedrop-admin tailsconfig (used for configuring desktop shortcuts and torrc additions in the SecureDrop Tails workstations) to:

• Use v3 addresses if v3 files are present for desktop shortcuts, else continue to use v2
• Add v3 Client auth details to torrc if they are present (and NOT add v2 HidServAuth so that it’s easier in the future to rip out v2 without making a round of updates in journalist and admin drives)

Subticket of #2951

[zenmonkeykstop]

So it looks like in order to do this, the client-side changes are:

1. Create the ClientOnionAuthDir and add an entry to to torrc specifying its location
2. add files <descriptor>.auth_private containing a single line like:
   <AddressWithoutDotOnion>:<descriptor>:x25519:<Base32PrivateKey>
3. restart Tor.

So the securedrop_init.py script will need to be modified to update torrc with that directive, and a task will need to be added to the playbook to create those single line files.

Substituting the onion addresses into the desktop shortcuts and ssh config will also need to be done, with some conditional logic for the cases v2 only; v2+v3; v3 only