Add new spacer content type

This allows us to model an empty space in the grid, to intentionally
leave gaps. This is necessary for both live content, to ensure we don't
mix live content and widgets in the same column, and also to introduce a
concept of pages

Bug: 378171351
Test: manually by adding empty spacer and verifying empty space in the
grid
Flag: EXEMPT spacer is not used anywhere yet

Change-Id: Ie3e42c1353c3fb29294ce97a388a29513a95a8a3