Cancel Button for pdf export

Problem Definition

Currently there is no option for canceling or killing already started PDF export.
Ability to cancel PDF export is required when

• PDF export of a large space is started accidentally
• Large PDF export is started on a server instance and resulted in performance impact or resource constraints
• Back button is clicked after PDF export started and export is initiated again by the user

Suggested Solution

An ability to cancel any ongoing PDF export process via WebUI.

Workaround

Confluence Data Center version has External Process Pool that minimises performance impact and potential system outages for large space export by letting external process crash without an impact on Confluence process. However, It doesn't offer an ability to cancel ongoing PDF export.
Confluence Server server version offers no option of killing space export to pdf task other than restarting the whole Confluence process.

Notes

On top of ability to cancel already started export, a confirmation dialogue and a warning about potential large space PDF export impact would also be beneficial and is tracked separately here: CONFSERVER-59553