Update issue templates: Add Bug report template

Author: todor-ivanov

.github/ISSUE_TEMPLATE/bug_report.md

@@ -0,0 +1,23 @@
+---
+name: Bug report
+about: Create a report to help us improve
+title: ''
+labels: bug
+assignees: ''
+
+---
+
+**Impact of the bug**
+Inform which external system gets affected by this bug. (e.g. CRAB3, WMCore or WMAgent/DBSUploader etc.) ?
+
+**Describe the bug**
+A clear and concise description of what the bug is.
+
+**How to reproduce it**
+Steps to reproduce the behavior:
+
+**Expected behavior**
+A clear and concise description of what you expected to happen.
+
+**Additional context and error message**
+Add any other context about the problem here, like error message and/or traceback. You might want to use triple back ticks to properly format it.