Dynamic voting window 🚀

[flibustier]

This PR bounces on suggestions made in #275

That is to say :

• The voting window is now the same as usual (this is now call as "initial_voting_window")
• Two reasons can now extend the voting window (call as "extended_voting_window") :
  « There are two scenarios that the voting window is needed to be long; where it is being hotly debated, and where it gets no votes for many hours. »
  And what is the value of the "extending_voting_window" ?
  The value depends of the time since the creation date of the repo, following this rule :

When the repository is less than 3 days, the extending voting windows is 3 hours
When the repository is more stable and advanced, 16 days or more, it's 9 hours

[chaosbot]

🙆‍♀️ PR passed with a vote of 11 for and 0 against, with a weighted total of 11.0 and a threshold of 6.2.

See merge-commit 23ca855 for more details.