Add "Load prior dates" button to empty planner
Add "Load prior dates" button to empty planner

Include the "Load prior items" hidden button on the empty planner.  If
it's needed on the planner for screenreader users, then it's needed in
an empty planner. It's also necessary so a selenium spec can click it.
It seems that sending page_up doesn't trigger the  past items query in selenium.
(see ADMIN-276)

fixes: ADMIN-591

Test plan:
  - have a user with a todo for yesterday
  - open the planner
  > expect to get the desert scene
  - TAB (see skip to main content
  - ENTER (to click button)
  - TAB until you're on the gear menu button
  - TAB once more
  > expect "Load prior dates" link to appear.
  - click it
  > expect yesterday's todo item to appear

Change-Id: Id986472e0d2abed11e8d480da53f8fae4e6df25d
Reviewed-on: https://gerrit.instructure.com/134396
Reviewed-by: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>
Tested-by: Jenkins
QA-Review: Dan Sasaki <ae19fabf7fe19dff310008c9b6a4d2ec84d311d8@instructure.com>
Product-Review: Christi Wruck

instructure/canvas-planner,instructure/canvas-planner,instructure/canvas-planner