Threat model update

[jskinne3]

Update the Workstation threat model according to these instructions

Threat modeling working group:

• @vickiniu
• @conorsch
• @zenmonkeykstop
• @deeplow

[conorsch]

The working group met yesterday and agreed on immediate next steps, including running a threat-modeling exercise under @redshiftzero with consult by @adaFPF. We also updated the existing historical threat model in spreadsheet form, so it's accurate in terms of the current Workstation architecture. When the upcoming security audit (#1441) kicks off, we'll include sharing the threat model with their team.

Post-audit, we'll summarize the detailed threat model in prose form, similar to what's already publicly available for the SecureDrop Server.

[jskinne3]

The working group expanded the threat model with App-specific threats with @adaFPF's help. We then designated threats in-scope and out-of-scope for the audit. We are mid-process reviewing it with the auditors.