fix(site): revamp UI for batch-updating workspaces (coder#18895)

Closes coder#18879
Builds on coder#18895

## Changes made
- Deleted `BatchUpdateConfirmation` component, replacing it with
`BatchUpdateModalForm`
- Added stories for the new component, trying to capture every variant I
could think of

## Screenshots
<img width="840" height="1059" alt="image"
src="https://github.com/user-attachments/assets/0fdac28e-19e8-4a14-a20a-fc1fd3758c81"
/>


## Notes
- There's too many problems to list, but look at the issue to see all
the problems we had with the old implementation
- It's definitely helpful to look at the stories to see all the things
the component is meant to cover

Author: buenos-nachos