Add project and repository creation requested events

Currently, Stash raises ProjectDeletedRequestedEvent and RepositoryDeleteRequestedEvent to allow plugins to perform cleanup when projects and repositories are deleted. It would be useful to raise similar events when a project or repository is being created.

Additionally, the existing events do not allow plugins to prevent projects or repositories from being deleted. Any exceptions they throw are caught, logged and swallowed by Atlassian Events. It could be very useful, especially for the new creation events, to allow plugins to cancel the operation and provide an explanation which would then be relayed back to the caller.