Resident agent installation key

**Is your feature request related to a problem? Please describe.**
Admin pass should not ever be supplied to non-admin devices. While installing resident agents, administrators may sign in on client devices, potentially leaking credentials and/or staying signed in to Remotely.

**Describe the solution you'd like**
- Discourage administrators from signing in just to download with a warning.
- Allow generating a mnemonic install key (passphrase), that can be typed in on the client device. Enforce an expiration date.

**Describe alternatives you've considered**
Downloading the files and distributing them by other means (filedrop may be insecure), such as USB.

The issue names no files, tests, or entry points. Start by locating the resident-agent installation and sign-in/download flows in the Remotely repository; completion should include a warning, an expiring mnemonic install key, and no need to expose an administrator password on client devices.

Written by the indexing model from the issue text.