Make automatic disabling of blocktype plugins more obvious

[Richard Mansfield]

When you disable an artefact plugin on the plugins page, any blocktypes associated with that plugin get automatically disabled, but the only clue you have is that the 'Hide' next to the blocktype changes to 'Show'. If you subsequently 'Show' the artefact plugin, the blocktypes remain hidden.

If blocktypes get hidden automatically, the green success message after hiding the artefact should be more explicit and say something like "You have hidden the foo artefact plugin and 3 associated blocktype plugins."

It would probably also be helpful if hidden plugins were displayed in a different colour.