api keys: rework API key UI.

* Add an explanation of what an API key is and how to use it.
* Make it possible for the site owner to view all API keys.
* Remove the requirement to re-enter your password before you can view
  your API key (to be reworked).
* Move the API key controller from maintenance/user/api_keys_controller.rb
  to a top level controller.

app/policies/api_key_policy.rb

@@ -0,0 +1,17 @@
+class ApiKeyPolicy < ApplicationPolicy
+  def create?
+    !user.is_anonymous?
+  end
+
+  def index?
+    !user.is_anonymous?
+  end
+
+  def destroy?
+    record.user == user
+  end
+
+  def api_attributes
+    super - [:key]
+  end
+end