[css-grid][css-multicol][css-flexbox][css-tables] Proposal for Various Gap Issues #1696

We've had some feedback on gaps, the one prompting this discussion being that authors are finding it awkward that the grid shorthand resets the gaps, preferring that it does not and that it is treated similar to spacing controls such as grid container padding and grid item margins. (#1036)

Unwinding the threads of discussions and resolutions, and discussing with several people at the F2F here, we have the following proposal:

• Disconnect the grid-gap properties from the grid shorthand, resolving [css-grid] Setting of gutter properties when using the 'grid' shorthand #1036
• Undo the resolution changing column-gap and row-gap to grid-column-gap and grid-row-gap.
  • As before that resolution, column-gap is shared with multi-col and normal computes to zero except on multi-column elements.
• Apply column-gap and row-gap to flex containers, resolving a strong demand from flexbox users to have this spacing control. (See Rachel's message to www-style and [css-flexbox] Make it easier to define margins that only apply between flex-items #592 with special note of erikjung's comment)
• As a clean-up measure, have these properties obsolete border-spacing (which inherits, which is weird) by having them defer to border-spacing when specified as normal and override it otherwise.