Merge BBS default reviewers with reviewers from auto add approvers listener

In BBS 4.8.0 default reviewers were added: https://jira.atlassian.com/browse/BSERV-2924

This does not work well with the auto add approvers listener. Currently the list of reviewers is overwritten with the ones from the listener.

We should merge the reviewers from the listener with the Bitbucket ones, so the ScriptRunner ones always take precedence (for the case of mandatory reviewers).