Add dashboard panel component

What does this MR do?

This MR adds the panel component for gitlab-org/gitlab-services/design.gitlab.com#1643 (closed) to GitLab UI. It is part of the effort to move dashboarding UI components from https://gitlab.com/gitlab-org/gitlab/tree/master/app/assets/javascripts/vue_shared/components/customizable_dashboard and to set a standard for dashboards to use across GitLab.

Screenshots or screen recordings

Scenario	Screenshot
Default
Title popover
Loading
Still loading
Actions
Custom border
Custom title icon
Alert

Integration merge requests

• GitLab: Draft: GitLab UI integration test for 2764-add-... (gitlab!174846 - closed)
• [-] CustomersDot: mr_url
• [-] Status Page: mr_url

Does this MR meet the acceptance criteria?

This checklist encourages the authors, reviewers, and maintainers of merge requests (MRs) to confirm changes were analyzed for conformity with the project's guidelines, security and accessibility.

Toggle the acceptance checklist

Conformity

• Code review guidelines.
• GitLab UI's contributing guidelines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• [-] If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened (see integration merge requests above).
• Added the ~"component:*" label(s) if applicable.

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] Security reports checked/validated by a reviewer from the AppSec team

Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.

Closes #2764 (closed)