[speaker] add confirmation message when answering a talk request

[EtienneDOYON]

Description

Add a confirmation alert/modal whenever a speaker accepts or declines their presence to a talk.
I am willing to do the PR if needed and if that small feature is accepted 😉

Additional context

As someone that is an absolute airhead, I recently mistakenly clicked on the button to decline my presence to a talk, instead of accepting said talk. I noticed my mistake as soon as I clicked, but it was already too late, and I had no way of cancelling 😅
This would help people like me, by giving them an extra step to doublecheck that they're doing the right thing.