Widen the scope of the unmerged branches metric to also allow for measuring merged branches that are not deleted

[fniessink]

Rename metric to "Inactive branches".

Add parameter "Merge status" with values "unmerged branches", "merged branched", "both".

In the migration code, set merge status to "unmerged branches", so that the metric keeps measuring the same value after the migration.

Change default unit name to "inactive branch".