Add issue templates

.gitlab/issue_templates/Bug.md

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+**Please first check if another issue has been opened for your problem**
+
+## Versions
+
+  - OS: 
+  - Gajim version: 
+  - Plugin version: 
+  - GTK version: 
+  - Python-nbxmpp version: 
+
+## Steps to reproduce the problem
+
+  1.
+  1.
+  1.
+
+## Expected behavior
+
+
+## Actual behavior
+
+

.gitlab/issue_templates/Enhancement.md

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+**Please note by far the quickest way to get a new feature is to file a Merge Request.**
+
+## Description of the new feature
+
+