Adopt REST v2

[mvlasovatl]

New versions of Atlassian products will use REST v2, that will be incompatible with existing API. We'll need to adopt it in new versions of the plugins while still supporting older versions.

• Create plugin-specific release branches for maintenance of existing minor plugin versions
• Switch to atlassian-rest-v2-api library and modify the <Import-Package> section if necessary
• Change the package name of AnonymousAllowed
• Add tag in the atlassian-plugin.xml which contains the <rest> tag to enable REST v2 during migration period (migration period based on the timeline of each product).

<rest-migration key="rest-migration-key">
    <rest-v2/>
</rest-migration>

I may be delivered in Confluence 8.8. Let's get back to this task once there is a Confluence version to test against.