Add: "N available" copy to the dispatcher side.

[teesloane]

Describe your changes

Up for discussion: adding clarification copy about remaining slots to sign up for on the dispatcher side. Originally suggested up by @chadmohr in this slack thread.

Screenshot

https://github.com/bikebrigade/dispatch/assets/12987958/de19449a-440e-4eea-90a8-6a11f4f4b5f8

Checklist before requesting a review

• [X] I have performed a self-review of my code
• [ ] If it is a core feature, I have added tests.
• [ ] Are there other PRs or Issues that I should link to here?
• [ ] Will this be part of a product update? If yes, please write one phrase about this update in the description above.