Ability to Merge Profiles

**User story**
> As an administrator, I want to easily merge multiple profiles into one in order to delete duplicates and automatically reassign corresponding posts.

Use case from CPJ: https://alleyinteractive.atlassian.net/browse/CPJD-551

**Description**
If a site contains duplicate profiles (after a migration, for example), it would be nice to be able to merge them together to clean up the duplicates, and automatically reassign post bylines to the merged profile. The functionality would be similar to the way the [term management plugin](http://scribu.net/wordpress/term-management-tools/) works.

**Example UI**

_Add "Merge" option to bulk actions on Profile list view_

_Add an autocomplete for "Merge into" field_

_After merge: the merged profiles are deleted, and associated posts are reassigned to new profile_