Allow bulk deletion of activities

[emmajclegg]

User story: "As a user, I may want to delete a group of activities at once, rather than one by one, to save time"

Motivations:

• As users can bulk import and publish activities, it is intuitive that they should also be able to bulk delete activities too. This may be needed, for example, if they have imported duplicate activities by mistake.
• Deleting activities one by one in the interface is labour-intensive. Currently, an end-user would need to contact IATI Support if they needed to remove a large group of activities from IATI Publisher.

Considerations:

• Users should not be able to delete an activity from the interface without unpublishing it first
• Activities should remain public once published. Users should not be encouraged to delete old activities.

To discuss: what the bulk deletion limit should be (10 may be reasonable)