GitLab for Slack app notifications: UX for private Slack channels

This issue came out of this discussion #373321 (comment 1178388844).

Problem

Our GitLab Slack app needs to be "invited" or added to a private Slack channel first before we are able to send a notification to the channel. Public channels we can post to without the app being added.

Slack provides a few ways for an app to be added to a channel, and the UX for all of them is fairly good:

• /invite @GitLab.
• /add and then choose our GitLab app.
• Mention them: @GitLab and then choose Invite Them.

It's a one-off requirement for private channels only.

We need to make people aware of this limitation so they don't experience the integration as being broken.

Proposal

1. Update the GitLab for Slack app integrations form page (https://gitlab.com/<PROJECT>/-/settings/integrations/gitlab_slack_application/edit) to display an In-page info alert that describes the need to invite the GitLab bot to private channels in order to receive notifications.
2. Update the App Home to include the same information

Note Docs already include this information !104639 (merged).

Design source

• ✏ Figma
• 💻 Slack Block kit

Availability & Testing