Add idea issue template

.gitlab/issue_templates/idea.md

+<!--
+This template should be used for proposing a new RFC idea, discussing it and
+potentially finding collaborators to work on a merge request (the eventual
+RFC).
+-->
+
+# Idea:
+<!--
+Add the title after the "Idea: "
+-->
+
+## Summary
+<!--
+Add a short summary.
+-->
+
+## Specification
+<!--
+If there is a more defined specification already, add it here.
+-->
+
+## Drawbacks
+<!--
+If there are any known drawbacks, add them here.
+-->