perf: prevent an unneeded db/redis query for chat tool

test plan:
* for courses that have the chat LTI tool enabled it should show up
  exactly as before

* for courses that have no lti tools, profile a page load,
* make sure you don't see anything like this in the logs:
Redis (0.261ms) get active_external_tool_for/courses/6961-xyz/chat
SQL ContextExternalTool Load (1.0ms)  SELECT "context_external_tools"...
Redis (0.253ms) get shard_7549:account_chain_ids/accounts/7549000...
SQL ContextExternalTool Load (0.9ms)  SELECT "context_external_tools"...
Redis (0.295ms) set active_external_tool_for/courses/6961-xyz/chat

Change-Id: Ia660a6f2eee6d08a3aabca9fa6643efd82b21040
Reviewed-on: https://gerrit.instructure.com/210798
Tested-by: Jenkins
Reviewed-by: James Williams <jamesw@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>

app/helpers/application_helper.rb

@@ -352,8 +352,8 @@ module ApplicationHelper
       !@body_class_no_headers &&
       @current_user &&
       @context.is_a?(Course) &&
-      embedded_chat_enabled &&
-      external_tool_tab_visible('chat')
+      external_tool_tab_visible('chat') &&
+      embedded_chat_enabled
   end
 
   def active_external_tool_by_id(tool_id)
@@ -379,9 +379,10 @@ module ApplicationHelper
   end
 
   def external_tool_tab_visible(tool_id)
+    return false unless available_section_tabs.any?{|tc| tc[:external]} # if the course has no external tool tabs, we know it won't have a chat one so we can bail early before querying the db/redis for it
     tool = active_external_tool_by_id(tool_id)
     return false unless tool
-    @context.tabs_available(@current_user).find {|tc| tc[:id] == tool.asset_string}.present?
+    available_section_tabs.find {|tc| tc[:id] == tool.asset_string}.present?
   end
 
   def license_help_link

app/helpers/section_tab_helper.rb

@@ -34,9 +34,9 @@ module SectionTabHelper
     :view_all_grades
   ].freeze
 
-  def available_section_tabs(context, precalculated_permissions=nil)
-    AvailableSectionTabs.new(
-      context, @current_user, @domain_root_account, session, precalculated_permissions
+  def available_section_tabs
+    @available_section_tabs ||= AvailableSectionTabs.new(
+      @context, @current_user, @domain_root_account, session
     ).to_a
   end
 
@@ -56,13 +56,13 @@ module SectionTabHelper
 
   def section_tabs
     @section_tabs ||= begin
-      if @context && available_section_tabs(@context).any?
+      if @context && available_section_tabs.any?
         content_tag(:nav, {
           :role => 'navigation',
           :'aria-label' => nav_name
         }) do
           concat(content_tag(:ul, id: 'section-tabs') do
-            available_section_tabs(@context).map do |tab|
+            available_section_tabs.map do |tab|
               section_tab_tag(tab, @context, get_active_tab)
             end
           end)