Make it easier to add items to a campaign

[clarkepaul]

Through user testing we observed that participants wanted to add items from the campaign itself (rather from a page), this would depend on which context they would be coming from (campaign or page). They also struggle to find the link from pages/files so this would also be a good thing to look at improving if possible.

Access to the Treedropdown field for both pages and files could be a way to improve this functionality.

[chillu]

There's instructions for how to add items on the "new campaign" screen - I think that's good enough for now. It's going to be non-trivial to allow users to select arbitrary objects from SilverStripe to add to a campaign in a useful fashion. Removing from milestone.

[clarkepaul]

I've changed the impact level to reflect the addition of the designs implemented as mention by @chillu in the previous comment. I've noted this as something which could do with additional testing once people are using the feature more.