Warn about conflicting changes in other pending Merge Requests

Description

The problem / use case is when two or more feature branches (eg. in GitLab Flow) have conflicting changes - you sometimes won't realize that until one is merged into master - or even until you try to merge the second one. There are of course different approaches (Continuous / Promiscuous Integration, ...), but that's a different topic.

Proposal

The idea of this proposal is to check for conflicting changes between pending Merge Requests, and display them in the corresponding MRs. Another possibillity would be to do this with branches - but not all branches are meant to be merged back into their originating branch, that's why I propose to do this with MRs, as you know their goal is to be merged into a specific branch.
Bonus: This would also encourage people to use 'Work in Progress' MRs.

~"feature proposal"