Add labels to issue templates

Signed-off-by: Julius Härtl <jus@bitgrid.net>

.github/ISSUE_TEMPLATE/Bug_report.md

 ---
 name: Bug report
 about: Help us improving by reporting a bug
+labels: bug, 0. Needs triage
 ---
 
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.github/ISSUE_TEMPLATE/Feature_request.md

 ---
 name: Feature request
 about: Suggest an idea for this project
+labels: enhancement, 0. Needs triage
 ---
 
 <!--