No colour indicating changes between blocks on pages

[clarkepaul]

When using Elemental blocks on pages, versioned admin is not highlighting the changes between blocks, we would expect to see the indications of deleted, and new text.

Version tested with:
SilverStripe 4.3.1 (CWP 2.2)

Colours are showing on Page changes (although green is almost impossible to see, there is already an older issue for that)

CC @brynwhyman, @robbieaverill

[brynwhyman]

I haven't tested this with older versions, but I expect this is a regression

[sachajudd]

Refer to #97 for designs.