Ask for the current password to change it

Changing the password now ends every other session, so whoever holds
one could shut the owner out of their own account with a new password.
The profile asks for the current password whenever the password
changes; without it, or with a wrong one, nothing is saved and the page
says why. Other profile edits don't need it, and the admin
reset-password script still works without it.

Attempts are rate limited like signing in, 10 in 3 minutes per address,
so the current password can't be guessed here instead.

test/script/admin/reset_password_test.rb

@@ -0,0 +1,21 @@
+require "test_helper"
+
+class ResetPasswordScriptTest < ActiveSupport::TestCase
+  test "resets a password without the current one and signs out every session" do
+    assert_output "Password has been reset\n" do
+      run_script "david@37signals.com", "a new secret123456"
+    end
+
+    assert users(:david).reload.authenticate("a new secret123456")
+    assert_empty users(:david).sessions
+  end
+
+  private
+    def run_script(*arguments)
+      original_arguments = ARGV.dup
+      ARGV.replace(arguments)
+      load Rails.root.join("script/admin/reset-password").to_s
+    ensure
+      ARGV.replace(original_arguments)
+    end
+end