Fix Authorize user page for Mattermost

What does this MR do and why?

The "chat name" authorize page is shared between Mattermost and the GitLab for Slack app integrations.

In !113801 (merged) the copy was changed to only reference Slack.

This change identifies the integration by the params to now also mention Mattermost correctly.

We can only do this because Slack and Mattermost use very different-looking IDs from each other. Mattermost's are lowercase and Slack are uppercase, and additionally Slack are reliably prefixed !116492 (comment 1338071973).

#395503 (closed)

Screenshots or screen recordings

GitLab for Slack app	Mattermost

Recordings

GitLab for Slack app	Mattermost
Screen_Recording_2023-04-03_at_11.10.47_AM	Screen_Recording_2023-04-03_at_11.41.18_AM

How to set up and validate locally

Fully validate it

To fully test this out, members of ~"group::integrations" and ~"group::import" can set up a GitLab for Slack app integration according to this guide. Remember to change to this branch.

Anyone can sign up for a Mattermost Cloud account and configure the integration manually.

Then, from the either the development copy of our Slack app, or Mattermost Cloud: type: /gitlab help, and follow the link to authorize your account.

Quickly test it out

1. On the rails console:

      params = { team_id: 'T00', team_domain: 'my_chat_team', user_id: 'U01', user_name: 'my_chat_user' }
      ChatNames::AuthorizeUserService.new(params).execute

2. Follow the URL generated to see the GitLab for Slack app authorize page.
3. Generate a second URI using non-Slack looking params.

      params = { team_id: 'anything_else', team_domain: 'my_chat_team', user_id: 'something_else', user_name: 'my_chat_user' }
      ChatNames::AuthorizeUserService.new(params).execute

4. Follow the URL generated to see the Mattermost authorize page.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #395503 (closed)