Enable the Featured Conference feature

[HedicGuibert]

In the User Account, we prepared for the Featured Conference feature, but we currently have no way to tag a Conference as Featured.

Imo, we want to have the possibility to tag a conference as Featured in 2 places :

• A second button in Slack notifications (maybe only allow admins to use it ?)
• In the admin directly

A pretty cool thing too would be to configure a fetcher to automatically tag all its conferences as featured. For exemple, we at JoliCode would probably want to have all Symfony conferences as Featured, so we would configure the SymfonyFetcher to set all its conferences as Featured.

BIG maybe, but maybe we would want to also do it depending on the conference tag ? Like auto set any conference with the php tag as Featured.