Update slack at here command for the design channel

[dingo_d]

This ticket addresses the request to add @mapk to the list of people who can announce on slack in the design and the core-editor channels.

[coffee2code]

In 9093:

Slack: Grant announce privilege to mapk for #design, per karmatosed.

Props dingo_d.
See ​https://wordpress.slack.com/archives/C02QB8GMM/p1565108699206600.
Fixes #4660.

[karmatosed]

Thanks for resolving this @coffee2code and for creating ticket @dingo_d.