properly detect sessions plugin

fixes CORE-2320

check if it's enabled, not if it has (possibly inherited) settings

test plan:
 * have canvas auth, don't set up the sessions plugin
 * /login/canvas should have a remember me checkbox

Change-Id: Ia29d0f4a4cd7a8bd04f58ad331f01303f5aa8647
Reviewed-on: https://gerrit.instructure.com/180627
Tested-by: Jenkins
QA-Review: Tucker Mcknight <tmcknight@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
Reviewed-by: James Williams <jamesw@instructure.com>

app/helpers/login/canvas_helper.rb

@@ -18,7 +18,7 @@
 
 module Login::CanvasHelper
   def session_timeout_enabled?
-    PluginSetting.settings_for_plugin 'sessions'
+    PluginSetting.cached_plugin_setting('sessions')&.enabled?
   end
 
   def reg_link_data(auth_type)

spec/helpers/login/canvas_helper_spec.rb

@@ -20,22 +20,19 @@ require_relative '../../spec_helper'
 
 describe Login::CanvasHelper do
   describe "#session_timeout_enabled" do
-    context "when the sessions plugin is enabled" do 
-      before do 
-        expect(PluginSetting).to receive(:settings_for_plugin).with('sessions').and_return({"session_timeout" => 123})
-      end
-
+    context "when the sessions plugin is enabled" do
       it "returns true" do
+        ps = PluginSetting.new
+        ps.name = 'sessions'
+        ps.disabled = false
+        ps.settings = {"session_timeout" => 123}
+        ps.save!
         expect(helper.session_timeout_enabled?).to be_truthy
       end
     end
 
-    context "when the sessions plugin is disabled" do 
-      before do 
-        expect(PluginSetting).to receive(:settings_for_plugin).with('sessions').and_return(nil)
-      end
-
-      it "returns false" do 
+    context "when the sessions plugin is disabled" do
+      it "returns false" do
         expect(helper.session_timeout_enabled?).to be_falsey
       end
     end