Update slack api endpoints to remove depreciated channels.* methods

[alexeiser]

Slack has depreciated all of the channels.* methods to migrate to the conversation ones.

https://api.slack.com/methods/channels.history
https://api.slack.com/methods/channels.join
https://api.slack.com/methods/channels.list
https://api.slack.com/methods/channels.info
https://api.slack.com/methods/channels.archive

These methods are deprecated. They will stop functioning in February 2021 and will not work with newly created apps after June 10th, 2020.
https://api.slack.com/changelog/2020-01-deprecating-antecedents-to-the-conversations-api

https://api.slack.com/methods/conversations.history
https://api.slack.com/methods/conversations.join
https://api.slack.com/methods/conversations.list
https://api.slack.com/methods/conversations.info
https://api.slack.com/methods/conversations.archive

[alexeiser]

NOTE: that newer slack apps - can not use the post as bot feature from this repo. This means that messages are posted as the user who created/added the app - and not as channel_reaper. While https://github.com/gareth-sharpe/slack-autoarchive makes an attempt at migrating to a bot only flow - the implementation has small issues (mostly because a user auth token is no longer needed at all)

[budbach]

@alexeiser This is sorely needed but without handling pagination (https://api.slack.com/docs/pagination), the implementation of get_all_channels() will be pretty broken I think. By default, https://slack.com/api/conversations.list returns only the first 100 items (https://api.slack.com/methods/conversations.list).

[alexeiser]

@alexeiser This is sorely needed but without handling pagination (https://api.slack.com/docs/pagination), the implementation of get_all_channels() will be pretty broken I think. By default, https://slack.com/api/conversations.list returns only the first 100 items (https://api.slack.com/methods/conversations.list).

Agreed - and I borrowed #42 to use on my branch of this repo.

[egeland]

February 2021 is getting really close... any plans to merge this PR?