FEAT: Allow user to manually pick time and attendees

[gcharest]

Right now the bot searches for the earliest free time slot available in all the users in the channel and if it finds one, it'll automatically create the event.

• As a user, I should be able to preview the date/time found and confirm before creating the event.
• if that date/time doesn't work, I should be able to pick another time off a list of available times before creating the event.