Add a confirmation dialog before moving to action items phase

[marciobarrios]

Yesterday we received this email:
"We accidentally went forward to the action item phase -- can we go back to the voting stage?",
It's from a long retro with more than 30 topics, I think it makes sense to add a confirmation modal when switching to the Add action items phase to avoid these situation.

It's easy and in my opinion it does not warm the user experience too much (it's not ideal though), in fact we will be consistent because we are showing a confirmation modal when going from the Brainstorm step to Group & vote.

What do you think? I think we can give it a try, it's something easy to implement. It's true that we already discussed something related in #41, but we didn't reach a clear agreement. What was clear is that we need to improve the instructions also.

[uesteibar]

@marciobarrios I think we should move forward with this, we've kept receiving feedback on it over and over

cc/ @rudeayelo @atrzaska