Reduce padding of blocks

I created MDL-63102 but did a very poor job of describing the issue and instead of reducing the padding on the blocks we reduced the margin between the blocks.

Currently the blocks are styled as cards and the default padding for the card body is 1.25rem. This adds quite a bit of whitespace to the block, particularly when it's in the side column where there isn't much room.

We can reclaim a little bit of that space for the content (and make the block look better overall) by reducing that down. We can use the `p-3` class that comes with Bootstrap 4 to avoid adding new custom CSS.

This issue can also revert the reduction to the margins that were introduced in MDL-63102.