Remove and add assignees to issue from Slack

• After https://gitlab.com/gitlab-org/gitlab-ce/issues/32164 is implemented, add buttons to remove and add assignees to an issue from within Slack.
• These buttons should be part of the JSON response that is rendered when displaying an issue in Slack from the two methods of #32164 (moved).
• Use the message button (and other interactive message menu) functionalities in Slack.
• When a user clicks a button (or otherwise interacts) with the UI, a payload is sent back to GitLab, and the response is sent back to Slack.
• Provide some feedback mechanism inside Slack to indicate success / failure.
• There is no straightforward mechanism inside Slack to determine if you have permissions to remove / add assignees. So the UX flow should just assume you have permissions when you attempt to make the change. The response from GitLab should fail and indicate you don't have permissions accordingly.
• When you interact and GitLab doesn't recognize you, Slack-GitLab should initiate an authorization flow allowing you to associate your Slack account with your GitLab account.

Helpful resources for designers:

• https://api.slack.com/docs/messages/builder
• https://api.slack.com/docs/message-menus