Split dashboard selenium specs out into four files

Refs CNVS-15688

One each for the main dashboard, teacher dashboard, todo section,
and coming up sections.

Test Plan:

Verify that all 4 files run in under two minutes each, on Jenkins.

Change-Id: I417eeb43a1c80a5134295459555b26e4a34e0231
Reviewed-on: https://gerrit.instructure.com/41578
Reviewed-by: Brad Horrocks <bhorrocks@instructure.com>
Product-Review: Matthew Wheeler <mwheeler@instructure.com>
QA-Review: Matthew Wheeler <mwheeler@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>