Add template for Gitlab issues

.gitlab/issue_templates/Bug.md

+## Description of problem
+<!--
+Describe in detail what you are trying to do and what the result is.
+Exact timestamps, error tracebacks, and screenshots (if applicable) are very helpful.
+-->
+
+## Expected behavior
+<!-- What do you expect to happen instead? -->
+
+
+## Steps to reproduce
+<!-- Step-by-step procedure for reproducing the issue -->
+
+
+## Context/environment
+<!--
+Describe the environment you are working in:
+    * If using the ligo-gracedb client package, which version?
+    * Your operating system
+    * Your browser (web interface issues only)
+    * If you are experiencing this problem while working on a LIGO or Virgo computing cluster, which cluster are you using?
+-->
+
+## Suggested solutions
+<!-- Any ideas for how to resolve this problem? -->