Add option to send notifications when all dependency MRs of an MR has been merged

Problem to solve

Merge request dependencies are a great way to ensure changes go in across projects in a specific order. However, right now the only way to know if an MR is merge-able (i.e, all its dependencies have been merged) is to periodically check the MR. This is a bit inefficient.

Proposal

Send email notifications when all the dependencies are satisfied for an MR.