Paginate through slack channels

[sej3506]

Problem

If there are over 1000 slack channels in an organization, which is
entirely possible with the ability to archive channels, the call method
in loads_slack_channels would only retrieve the first 1000.

Solution

This change adds a method to follow any pagination cursors and combine
channel list pages together to ensure we have a complete channel list.

This change also makes use of the exclude_archived argument for
conversations_list which will (on Slack's side) throw out any archived
channels. A snippet to keep in mind from Slack's docs:

"When paginating, any filters used in the request are applied after
retrieving a virtual page's limit. For example. using
exclude_archived=true when limit=20 on a virtual page that would contain
15 archived channels will return you the virtual page with only 5
results. Additional results are available from the next cursor
valuePaginate through slack channels."

[tuxagon]

@sej3506 apologies for such a long time before anyone responded (mostly my fault). This project went through a whirlwind of initial changes based on some internal experimentation we were doing and the repo happened to be public for GitHub Actions. I didn't initially have the extra bandwidth to give the project support with external contributions and I let this fall through the cracks.

The two PRs you worked on are valuable; they simply became something I kept putting off responding to for much longer than I anticipated. If you're still interested in getting these merged, they'll need updated. I will ensure they get merged this time around. No worries if you're no longer interested.