Duplicate pages service alerts in #gitlab-pages slack channel

From slack thread https://gitlab.slack.com/archives/C101F3796/p1588233444352000:

Is there a way to post alerts to #gitlab-pages channel about pages too? The pages development team lives there, and we check it much more often.

cc @grzesiek @krasio @sean_carroll @jaime @igorwwwwwwwwwwwwwwwwwwww