feat: admin portal for managing announcements from a single location

[kurtaking]

Checklist

• [ ] I have updated the necessary documentation
• [ ] I have signed off all my commits as required by DCO
• [ ] My build is green

Summary:

New announcement portal to create, edit, and delete announcements from a single view.

Details:

The current implementation displays announcements as cards on a single page. "Admins," or those with permissions to crud announcements, will see additional buttons to manage the announcements.

When a user clicks to create a new announcement, they are taken to a new page to create the announcement and then are redirected back on creation. The behavior is similar for editing announcements.

This PR introduces a new /admin route with a component to create, edit, and delete announcements from a single portal. You can leverage the permissions that are included with the plugins to lock down this route however you choose.

Consuming and managing announcements are two different concerns, and separating them from a single component will allow us to focus on them as two separate features.

wip

There is a flickering effect due to the way components are being rendered. This is something that must be fixed.

https://github.com/procore-oss/backstage-plugin-announcements/assets/14222009/41386708-1be3-4fad-b76e-5ca3ed514d1e

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Latest commit: a0ccebec9b2985a27018870f2475e88bbbb109f5

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