[Fetcher] Confs.tech fetchers duplicates conférences with multiple topics

[welcoMattic]

confs.tech lists conferences in json files, one per topic. Starfleet fetches those files, but does not unique the list of fetched conferences. If a conference is listed in 3 confs.tech json files, it will be added 3 times in Starfleet.

Todo:

• Handle this specific case in ConfsTechFetcher

Example:

https://starfleet.jolicode.com/admin/?action=search&entity=NextConference&sortField=startAt&sortDirection=ASC&menuIndex=1&submenuIndex=-1&query=codeurs+en+seine