[XDCR] Inconsistency between UI and CLI for migration mode

If user wants to enable migration mode via CLI/REST, here are the steps to be followed:

1. User should set collection-explicit-mappings to 1 first
2. User should set collection-migration-mode to 1
3. User can now set collection-explicit-mappings rules in migration mode format e.g.

If user tries to enable migration mode without turning explicit mapping on, an error is thrown:

[root@s83010 bin]# ./couchbase-cli xdcr-replicate -c 172.23.104.105 -u Administrator -p password --settings --xdcr-replicator 5c98cb76f095898f64b5c51b377b2498/travel-sample/default --collection-migration 1

ERROR: PreUpdateValidate - Migration must be used with explicit mapping

But in UI, the behavior deviates - explicit mapping toggle and migration toggle are mutually exclusive - if explicit mapping is toggled on, migration mode is toggled off automatically and vice versa - if migration mode is toggled on, user is given a text box on the source side where they are expected to enter migration format rules - if explicit mapping is on, they are showed the scopes and collections to choose from

This was discussed over an email thread (attached) - looks like engineering and PM agrees to change the UI to be consistent with CLI - assigning this to Shivani Gupta to take a decision on how to go about this