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

scripts/setup/manage_user.php

@@ -6,8 +6,8 @@ require_once $root.'/scripts/__init_script__.php';
 
 $args = new PhutilArgumentParser($argv);
 $args->setSynopsis(<<<EOSYNOPSIS
-**people** __command__ [__options__]
-    Manage user profiles and accounts.
+**user** __command__ [__options__]
+    Modify user accounts to regain access to an install.
 
 EOSYNOPSIS
   );