`/announce` and `/here` do not work for newer users

[dd32]

As reported in Slack #meta ​https://wordpress.slack.com/archives/C02QB8GMM/p1536701029000100 some users are unable to use the /announce and /here commands despite being whitelisted.

This is because Slack changed awhile ago to deprecate the user_name field: ​https://api.slack.com/changelog/2017-09-the-one-about-usernames

As done elsewhere in our Slack API code, the proper thing to do here is to determine the user by the slack user_id and convert that back into a WordPress user_login before comparing it to the whitelists.

[dd32]

In 7661:

Slack: When comparing a slack user to the user whitelist, determine the user by comparing the Slack ID to our slack database prior to using the deprecated user_name slack field.

This should allow users who sign up after a certain date to be whitelisted for /announce and /here commands.

See #3799.

[dd32]

In 7662:

Slack: Correct r7661 to use a DB query to get the user_login as WordPress isn't loaded for these APIs (although $wpdb exists).

See #3799.

[dd32]

In 7663:

Slack: Update the Announce whitelists to consistently use WordPress.org usernames rather than non-unique deprecated slack user_names.

See #3799.

[dd32]

Based on feedback in ​https://wordpress.slack.com/archives/C02QB8GMM/p1536760595000100 I'm going to close this as working.