Prevent focussing items until retro is "started"

[davidje13]

While adding items or voting, it is easy for people to accidentally click on (and focus) an existing item, which can cause confusion.

Possible fixes:

• add a toggle to the page for enabling item focus, which defaults to off. This could be switched on per-device, allowing the main facilitator to interact while other users still cannot focus items
• add a "start" button to the page, which changes the shared retro state. This might be more intuitive, but would risk other people affecting the retro once it has begun
• use another gesture to focus items, such as a double-click (might be better to use double-click-to-edit though)