Slack notifications, slash commands, and unfurled links should show same content when displaying issue

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Description

• Currently, when you (1) use a slash command to create / display an issue, (2) receive a Slack notification, and (3) when you paste an issue link to display an issue, it shows different information in a different content.
• The slash command being: /gitlab-ce issue show 123 for example.
• This issue will make sure the same information is displayed in the same format in all three scenarios.
• GitLab should serve the same information back to Slack in a common format, because in another issue, we will be adding interactive buttons, and we want to ensure that we get a consistent UX experience. This is also a best practice per Slack's suggestion.
• We need UX to review the existing designs, and provide a consistent design, along with the message button UX flow (https://gitlab.com/gitlab-org/gitlab-ce/issues/32165).
• The scope of this issue is just making a common design for the two scenarios. https://gitlab.com/gitlab-org/gitlab-ce/issues/32165 starts adding the message buttons, but is not part of the scope here.
• When a command or link is pasted in Slack, a payload of information is sent back to GitLab, which processes it, and sends back a JSON response, which Slack renders. Designers can refer to the message builder here to see what is possible: https://api.slack.com/docs/messages/builder.