New scoring system: from 👍/👎 to approval and unresolved threads

Following the discussions on the previous workshop, the idea is to:

• Remove the use of 👍/👎 to compute the MR score
• The 👍 is replaced with gitlab Approval system, which is more integrated/flexible in gitlab (both in UI and in the workflow). The approval validity remains the same as the 👍 votes:
  • You cannot approve your own MR (the bot will not take it into account)
  • Only users with a developer+ role will be taken into account
• The 👎 is replaced with the existing unresolved thread rule.
  • Warning: this action can be very volatile, as the MR owner can resolve any thread included in his/her own MR. This is also a common habit among developers that are used to github platform: closing a discussion is a synonym of "I have made the changes you requested. Feel free to reopen the discussion if you disagree."
  • This implies either to be swiftly reactive to MR notifications and/or to be more cautious on Approval delivery.