Add themereview leads to list of slack announcers

[dingo_d]

Currently the themereview leads cannot use @here or @channel commands to announce the meetings.

This has caused some people to miss out on the meetings.

I have a patch ready that adds current team leads to the list of announcers.

[dingo_d]

Add williampatton and kafleg to the list of announcers

[SergeyBiryukov]

In 9034:

Slack: Give announce to kafleg and williampatton in #themereview.

Props dingo_d.
Fixes #4591.