Replace "bin/people profileimage" with "bin/user enable|empower"

Summary:
Ref T13382.

• Remove "bin/people profileimage" which previously generated profile image caches but now feels obsolete.
• Replace it with "bin/user", with "enable" and "empower" flows. This command is now focused on regaining access to an install after you lock your keys inside.
• Document the various ways to unlock objects and accounts from the CLI.

Test Plan:

• Ran bin/user enable and bin/user empower with various flags.
• Grepped for people profileimage and found no references.
• Grepped for bin/people and found no references.
• Read documentation.

Maniphest Tasks: T13382

Differential Revision: https://secure.phabricator.com/D20724