Allow editors to create new events with custom header, body, and link.

[tsmyre]

Feature Description

It would be useful to be able to manually create Activity Feed tiles, either to make general announcements for the project or to highlight new materials that don't normally draw a new tile (e.g., adding material to existing texts through reingestion).

Why is this feature important? Who does it help?

Currently, events are only created programatically. Allowing admins to manually create events would give them a way to share project updates and news.

User Stories

As a user, I want to be able to author an event with a custom title, body, and url.

Design Notes

No additional design is required. We can use existing event styles.

Development Notes

We may want to allow backend users to edit existing events, as well as custom events.