Split "Slack" service into "Slack notifications" and "Mattermost notifications" services

Description

The Slack service is compatible with Mattermost, but there's no mention of this compatibility.

Proposal

GitLab is expanding it's chat services (e.g. “Mattermost commands” service), so the current “Slack” service should be split into “Slack notifications” and “Mattermost notifications” services, to be more specific.

The service name, description, and help text should be changed to the ones shown in the designs. In the “Mattermost notifications” service, the “Webhook URL” field placeholder should be unique to Mattermost (also in the designs).

Use the following links in the setup text:

• Slack: “Add an incoming webhook integration”
• Mattermost:
• “Enable incoming webhooks”
• “Add an incoming webhook”

Designs

🔍 Check design specs (for spacing, sizes, colors and text copying)

“Mattermost notifications” service	“Slack notifications” service

MR review

Mention or assign @pedroms to the MR for UX review before ~Frontend review begins.