reverse the flow of columns into the grid
reverse the flow of columns into the grid

Managing columns for Slick Grid mixes different control flows.
This eliminates part of the complexity by putting all column
definitions into a single map and referencing them as needed by
their id.

Subsequent changes will further reduce the complexity.

refs CNVS-38346

test plan:
* ensure Jenkins passes

Change-Id: I091014895942706c7438e1c8d7a5b8390d983f58
Reviewed-on: https://gerrit.instructure.com/121508
Tested-by: Jenkins
Reviewed-by: Gary Mei <4dff5ebcff9ed76793b70b9470ac64791a16ed11@instructure.com>
Reviewed-by: Shahbaz Javeed <6692d7be3fa215e5483a121d6fb314def6049877@instructure.com>
Product-Review: Jeremy Neander <20252aad5efdc666f083d82ab63ed0f9f72a043e@instructure.com>
QA-Review: Jeremy Neander <20252aad5efdc666f083d82ab63ed0f9f72a043e@instructure.com>

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