Correct slack activity miscount from fetch_slack_activity

[daveoconnor]

A number for the messages in the boost-beast2 slack channel in the release reports that didn't make sense had been noticed.

fetch_slack_activity wasn't properly accounting for multiple pages so the last update time for a channel would be from the first page of results, meaning when it moved to newer pages the last updated time wasn't accurately reflected. would have been visible only on high traffic channels.This PR corrects that.

It also

• Improves debugging and comments
• Adds a management command to clear the slack activity.
• Adds a migration for an unmanaged db table that was missed in a previous PR.
• Adds gateway determination to the pycharm debugger library and upgrades the library.
• Adjusts the fetch slack activity iteration so we're not going to hit rate limits.

For this to work clear_slack_activity --confirm should be run on the server, then we can wait for the daily fetch_slack_activity run to execute as normal. It takes a long time, not worth forcing.

[gregjkal]

LGTM