Add OAuth support

Rather than having a service user to retrieve all the data, it would be better to have OAuth support, allowing each user to have their own view with their own permissions.

We should also see if it would be possible to integrate this support with a SSO used both for XWiki and Jira to make it effortless for the end user to view a page using a Jira Macro.

Ideally, this OAuth support should fallback to standard auth using a default service user when the user isn't authenticated with the Jira instance.