Make drag-n-drop overlay be consistently sized
Make drag-n-drop overlay be consistently sized

The overlay was being sized based on its parent div which
with few files or none at all was very small because of
the nature of flexbox stuff.  This makes it key off of
its grandparent div which is much larger and more consistently
sized.

closes CNVS-26630

Test Plan:
  - Go to files
  - In a folder that is otherwise empty drag a file to the files
    area.
  - The overlay should appear as soon as you enter the files area

Change-Id: I23418af0a8d0de01d98c629a93310a293fc630e8
Reviewed-on: https://gerrit.instructure.com/81975
Tested-by: Jenkins
Reviewed-by: Felix Milea-Ciobanu <9384aa226ac9d3237ed0bc52e18ffcb251ee0bf7@instructure.com>
QA-Review: Jahnavi Yetukuri <ed2b2672ccc2e59e56c0615df7f01b579202e014@instructure.com>
Product-Review: Clay Diffrient <9dff2e5c98626d20d2848250c411e8009465bb87@instructure.com>

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