No editing of harvested datasets

[rossjones]

If the is_harvested flag is set on a dataset, we shouldn't allow users to edit it. We should probably say why.

[gtdata]

@rossjones be good to see this in action - can you show me pls?

[rossjones]

It's hard to differentiate them at the moment in Alpha. It's more that, if someone edits some text in a dataset that was harvested, the next time we harvest it may overwrite the change.

It could be that we need to fix harvesting in beta to identify that there has been a change to the dataset, and instead of just overwriting, find some way to merge the two different metadata.

[gtdata]

@rossjones ok, so we'll need to create an 'alert' explaining what the user is/isn't allowed to do, I'll look into it