Show similar Merge Requests (if any) when trying to create a MR

When displaying the create MR form one should see that a similar MR is already opened. See a preview below:

To elaborate more on this topic at the moment a new MR form is displayed, if there are any similar MRs (same source project/branch, same target project/branch), we know that the creation will fail so we might want to hide the create form. If this is acceptable I can work on that.

cc @DouweM