Channels functions deprecated by Slack

[yonst]

Channels functions deprecated by Slack (more details here: https://api.slack.com/changelog/2020-01-deprecating-antecedents-to-the-conversations-api).

The recommended option to replace is the Conversation module but few changes have to be done.
One of the changes is that Channels functions worked also with the channel name and the conversation functions are working just with channel id.

also for some reason, in conversation.list now all the channels are showing up.