[bug] Interval Task can still be created with no items selected

[igorsimb]

Need to add some guard against creating Interval Task if no items selected, i.e. all boxes are unchecked.

Steps to reproduce:

• Uncheck all boxes
• Click "Create Interval"

Expected behavior: Some error asking to check some boxes

Actual behavior: Task is created

[igorsimb]

How this can be achieved.

• Enable Django messages
• In the create_scrape_interval_task view check that the number of selected_item_ids is > 0. If not, stop the function and return a message error.