Updating JEMH's issue event listener to listen for IssueEventBundle's (JEMH-6994)

JEMH will now dispatch IssueEventBundle's in addition to standard issue created events.

Currently JEMH's issue event listener only listens for the older style of issue events, we do not listen for bundles at all.

Bundles contain multiple events, so we would need to decide what to do with those. If an issue gets created with an assignee you get an Issue created and Issue assigned event in the bundle

API for single events has been deprecated since Jira 6.4.10:

• https://docs.atlassian.com/software/jira/docs/api/8.3.2/com/atlassian/jira/event/issue/IssueEventManager.html

In addition to 'doing the right thing' in JEMH event listener, we also need a JEMH feature selector to allow customers to choose single IssueEvent or BundleEvent handling.