give tinymce tables some basic dimensions, fixes CNVS-4987
give tinymce tables some basic dimensions, fixes CNVS-4987

restore some styles that got removed in the bootstrap commit, but make
them specific to tinymce tables (in edit mode)

test plan:
1. go into a rich text editor
2. add a table
3. the cells should be at least 20px wide, even if there is no content

Change-Id: Ibd0e40f0b73930db00f73382b3836e2f70286b7b
Reviewed-on: https://gerrit.instructure.com/19121
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
QA-Review: Cam Theriault <b4133c36b6d893d0872cc09e87b536d16aa2d241@instructure.com>
Reviewed-by: Eric Berry <b4e131942074b4e4dafdcc14a63e85b7d8e18577@instructure.com>
Product-Review: Jon Jensen <44f878afe53efc66b76772bd845eb65944ed8232@instructure.com>

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