management: Add change_password command.

Zulip identifies users by realm+delivery_email which means that the
Django changepassword command doesn't work well -
since it looks only at the .email field.
Thus we fork its code to our own change_password command.

zerver/management/commands/change_password.py

@@ -0,0 +1,69 @@
+import getpass
+from argparse import ArgumentParser
+from typing import Any
+
+from django.contrib.auth.password_validation import validate_password
+from django.core.exceptions import ValidationError
+from django.core.management.base import CommandError
+
+from zerver.lib.management import ZulipBaseCommand
+
+
+class Command(ZulipBaseCommand):
+    # This is our version of the original Django changepassword command adjusted
+    # to be able to find UserProfiles by email+realm.
+    # We change the arguments the command takes to fit our
+    # model of username+realm and change accordingly the
+    # logic inside the handle method which fetches the user
+    # from the database. The rest of the logic remains unchanged.
+
+    help = "Change a user's password."
+    requires_migrations_checks = True
+    requires_system_checks = False
+
+    def _get_pass(self, prompt: str = "Password: ") -> str:
+        p = getpass.getpass(prompt=prompt)
+        if not p:
+            raise CommandError("aborted")
+        return p
+
+    def add_arguments(self, parser: ArgumentParser) -> None:
+        parser.add_argument("email", metavar="<email>", help="email of user to change role")
+        self.add_realm_args(parser, required=True)
+
+    def handle(self, *args: Any, **options: Any) -> str:
+        email = options["email"]
+        realm = self.get_realm(options)
+
+        u = self.get_user(email, realm)
+
+        # Code below is taken from the Django version of this command:
+        self.stdout.write(f"Changing password for user '{u}'")
+
+        MAX_TRIES = 3
+        count = 0
+        p1, p2 = "1", "2"  # To make them initially mismatch.
+        password_validated = False
+        while (p1 != p2 or not password_validated) and count < MAX_TRIES:
+            p1 = self._get_pass()
+            p2 = self._get_pass("Password (again): ")
+            if p1 != p2:
+                self.stdout.write("Passwords do not match. Please try again.")
+                count += 1
+                # Don't validate passwords that don't match.
+                continue
+            try:
+                validate_password(p2, u)
+            except ValidationError as err:
+                self.stderr.write("\n".join(err.messages))
+                count += 1
+            else:
+                password_validated = True
+
+        if count == MAX_TRIES:
+            raise CommandError(f"Aborting password change for user '{u}' after {count} attempts")
+
+        u.set_password(p1)
+        u.save()
+
+        return f"Password changed successfully for user '{u}'"