[mattermost] Required parameter should be channel Id

The help text of Mattermost backend mentions the required parameter as Channel name.
https://github.com/chaoss/grimoirelab-perceval/blob/5b4f848989967a51a4e21be1a3f6277f009f16ad/perceval/backends/core/mattermost.py#L466

However, going through the source code and mentioned example in the Perceval documentation I think the required parameter is `channel id`.

I would also like to suggest an alternative. We can use the following API to get a list of channels and match the channel name to retrieve the channel id(as done in groups.io and gitter): https://api.mattermost.com/#tag/channels/paths/~1channels/get

Start with perceval/backends/core/mattermost.py around line 466 and compare the help text with the Mattermost example in the Perceval documentation. Check the linked Mattermost channels API documentation to determine whether channel names should be resolved to IDs. Done means the required parameter description and, if pursued, the channel lookup behavior are consistent with the documented usage.

Written by the indexing model from the issue text.