Add feature request issue template

.gitlab/issue_templates/Feature-request.md

+## Description of feature request
+<!--
+Describe your feature request!
+Is it a web interface change? Some underlying feature? An API resource?
+The more detail you can provide, the better.
+-->
+
+
+## Use cases
+<!-- List some specific cases where this feature will be useful -->
+
+
+## Benefits
+<!-- Describe the benefits of adding this feature -->
+
+
+## Drawbacks
+<!--
+Are there any drawbacks to adding this feature?
+Can you think of any ways in which this will negatively affect the service for any set of users?
+-->
+
+
+## Suggested solutions
+<!-- Do you have any ideas for how to implement this feature? -->