Add support for Discord

Many development communities are using [Discord](https://discord.com/) as synchronous communication system as a *modern version* of IRC or as an Slack alternative.

Wouldn't it be nice to explore [Discord API](https://discord.com/developers/docs/intro) to learn how to extract information about server channels, participants, conversations, ... and to build panels equivalent to current Slack support in GrimoireLab?

**Closing conditions**
[] Discord as GrimoireLab data source support evaluation

[] Perceval support to get data from conversations in channels in a server

[] Basic enrichment support to get data close to current Slack data (to reuse GrimoireLab Slack pannels if possible)