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/controllers/api_keys_controller.rb

@@ -0,0 +1,26 @@
+class ApiKeysController < ApplicationController
+  respond_to :html, :json, :xml
+
+  def create
+    @api_key = authorize ApiKey.new(user: CurrentUser.user)
+    @api_key.save
+    respond_with(@api_key, location: user_api_keys_path(CurrentUser.user.id))
+  end
+
+  def index
+    params[:search][:user_id] = params[:user_id] if params[:user_id].present?
+    @api_keys = authorize ApiKey.visible(CurrentUser.user).paginated_search(params, count_pages: true)
+    respond_with(@api_keys)
+  end
+
+  def show
+    @api_key = authorize ApiKey.find(params[:id])
+    respond_with(@api_key)
+  end
+
+  def destroy
+    @api_key = authorize ApiKey.find(params[:id])
+    @api_key.destroy
+    respond_with(@api_key, location: user_api_keys_path(CurrentUser.user.id))
+  end
+end