Add notice at the beginning of issue templates asking reporters to check for duplicates first

[ci skip]
Signed-off-by: Rémy Coutable <remy@rymai.me>

.gitlab/issue_templates/Bug.md

+Please read this!
+
+Before opening a new issue, make sure to search for keywords in the issues
+filtered by the "regression" or "bug" label:
+
+- https://gitlab.com/gitlab-org/gitlab-ce/issues?label_name%5B%5D=regression
+- https://gitlab.com/gitlab-org/gitlab-ce/issues?label_name%5B%5D=bug
+
+and verify the issue you're about to submit isn't a duplicate.
+
+Please remove this notice if you're confident your issue isn't a duplicate.
+
+------
+
 ### Summary
 
 (Summarize the bug encountered concisely)
@@ -56,3 +70,5 @@ logs, and code as it's very hard to read otherwise.)
 ### Possible fixes
 
 (If you can, link to the line of code that might be responsible for the problem)
+
+/label ~bug

.gitlab/issue_templates/Feature Proposal.md

+Please read this!
+
+Before opening a new issue, make sure to search for keywords in the issues
+filtered by the "feature proposal" label:
+
+- https://gitlab.com/gitlab-org/gitlab-ce/issues?label_name%5B%5D=feature+proposal
+
+and verify the issue you're about to submit isn't a duplicate.
+
+Please remove this notice if you're confident your issue isn't a duplicate.
+
+------
+
 ### Description
 
 (Include problem, use cases, benefits, and/or goals)
@@ -15,3 +28,5 @@
 3. How does someone use this
 
 During implementation, this can then be copied and used as a starter for the documentation.)
+
+/label ~"feature proposal"