Add labels in the notification header for issue and merge request creation

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Problem to solve

When you're subscribed on a label and a merge request is created with some labels attached on the form creation (/merge_requests/new) you'll receive a notification but the reasoning behind this (the label attached) is missing. The message header includes branch and assignee. This also affects issue creation.

Branches: source_branch → master
Assignee: John Smith

Proposal

Add the labels attached in the header if present.

Branches: source_branch → master
Assignee: John Smith
Labels: Create, P2, S2, UX 

Links / references

This is somewhat related to gitlab-ce#51485.