XDCR UI - Migration mode to support "explicit mapping" use case

This is an improvement specifically for UI to address a use case Shivani Gupta mentioned in the original ticket (MB-42856). The parent ticket has more information.

The request is that users will anticipate to be using the migration mode, but they do not want to specify a filter. They merely want to replicate from a source default collection to a specific target collection (essentially explicit mapping) but done from a context of "migration".

From the user's perspective, users should accomplish the above following under the "Migration mode" context. XDCR has provided a method to do so from the REST perspective. The last thing we need is be able to present this "context" in the UI.