Remove `/committers` command in Slack

[desrosj]

The /committers command in slack has long been superseded by the @ committers group. It was ​recently accidentally used and a few problems came to light.

1. It results in double pings because @ committers and the individual Slack handles are included.
2. The list is out of date.

[desrosj]

4053.diff​ removes /committers and updates the get_committers() list to be what I believe is currently accurate.

[desrosj]

4053.2.diff​ also updates core.trac so that the new committers have the Core Committer label on their comments.

[dd32]

In 8068:

Slack: Remove the /committers slash command handler as it's no longer needed.

The Slack slash integration has also been disabled.

Props desrosj.
See #4053.

[dd32]

In 8069:

Slack: Update the list of Committers for slash command whitelisting purposes.

Props desrosj, dd32.
See #4053.

[dd32]

In 8070:

Trac: Update the list of Committers to add their proper titles on Trac.

Props desrosj, dd32.
Fixes #4053.