support for identifying users that acknowledge and resolve from Slack

[lalitkapoor]

user_id in the log_entries is null if the user acknowledges/resolves from Slack.

To capture who is performing those actions I'm doing 2 things:

1. Storing the values from the channel object that come back from the request for log entries - which includes the slack user id
2. Syncing the list of users in Slack into a slack_users table

Now we can join the channel_user_id column in the log_entries table with the id column in the slack_users table.

If you want to enable the optional Slack integration you have to add the SLACK_TOKEN environment variable

FYI, This pr also includes the commits from #7

[CLAassistant]

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1 out of 2 committers have signed the CLA.

✅ lalitkapoor
❌ rnaud
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