Organize fact-checking rushes

## User story

To stimulate the contributors, we could organize fact-checking periods during which the reputation gains are doubled (**to be determined**). For instance, we can say: "if an action is performed at most 24h (**to be determined**) after the video was published, all the gains received for this action (through positive votes) are doubled".

Related to #59, we can split this period into two parts:

1. The reputation gained from statement collection is doubled (so that people first focus on collecting statements rigorously)
2. The reputation gained from upvoted (sourced, **to be determined**) comments is doubled