UX: Update GitLab Slack installation flow

See parent epic for more context.

Problem

The current version of the GitLab Slack app has not been updated for years and is not leveraging Slack's latest functionality. For example, once the GitLab Slack app is installed, it doesn't appear in the "App" list within the Slack workspace. The current version also still displays the older version of Slack's branding.

See existing installation flow:

📺 https://youtu.be/G0-qWwEstS4?t=85

Proposal

• Update /profile/slack/edit with...
  • latest Slack branding
  • more up-to-date documentation & visual example
  • improved project selector (Can we leverage the work done for the Jira Connect app?)
• Update /services/gitlab_slack_application/edit
  • Trigger a notification in Slack that the app was successfully installed
  • Display a success alert message on the page that the app was successfully installed
  • Overall, apply UI polish to the existing view to leverage the latest GitLab UI components

✏ Figma prototype

Todo

1. Ensure that the latest Slack branding is used https://gitlab.com/gitlab-com/legal-and-compliance/-/issues/609
2. Create first proposal
3. Solicit feedback form other designers and engineers and Slack counterparts
4. Iterate on designs based on feedback
5. Involve Technical Writing
6. Iterate on designs based on feedback from Technical Writing
7. Define success metrics / tracking strategy
8. Move to workflowplanning breakdown