Cancel date changes

Problem/Motivation

When a user updates the recurrence configuration for an event a confirmation message is shown warning the user that instances will be recreated.

The confirmation only has a single button "Confirm Date Changes". If the user is in doubt about the change, the form does not provide a clear way to cancel it.

Furthermore clicking the default "Save" button does not really do anything.

Steps to reproduce

1. Spawn a test site with the Recurring Events module on SimplyTest.me
2. Create an event series with the "Daily event" recur type
3. Set the start and end date for the schedule so 2 events are created
4. Save the event series
5. Edit the event series again
6. Change the end date for the event series to add at least one more date
7. Click "Save"
8. See that the "Confirm Date Changes" section only has a single button: "Confirm Date Changes"

Proposed resolution

- Add a "Cancel" button which reverts the changes to the recurrence configuration. It should preferably keep all other changes for the series.
- Either hide the "Save" button when the confirmation section is shown or make it do the same thing as the "Confirm Date Changes"

Remaining tasks

- Implement the solution

User interface changes

A cancel button is added to the confirmation section of EventSeriesForm.

API changes

None.

Data model changes

None.