Update new feature template to add user story requirement (#2166)

* Add user story to issue template

* Simplify feature request template

.github/ISSUE_TEMPLATE/Feature_request.md

@@ -4,51 +4,26 @@ about: Suggest a feature or enhancement
 
 ---
 
-## Motivation
+## User Story
 
 <!--
-What problem are we trying to solve?  Please link any relevant issues.
-What use cases are we trying to accommodate?
+This section should have a one-sentence sentence user story, such as:
 
-Focus on the problem and save design ideas for the next section.
--->
-
-## Design Alternatives
-
-<!--
-How could we accommodate the use cases above?
-Is "do nothing" an option?
--->
-
-## Design
+ As a <role> I can <capability>, so that <receive benefit>
 
-<!--
-Which design should we implement?
-What are the advantages of this design?
-What are some potential drawbacks of this design?
--->
-
-### Mock-Up
-
-<!--
-What will this design look like to developers?
-What will this design look like to end users?
+See also: https://en.wikipedia.org/wiki/User_story#Common_templates
 -->
 
-### Concepts
+## Rationale
 
 <!--
-How will we teach this design?
-What terminology will work best for the new concepts introduced by this design?
-What existing precedents support the new concepts?
-Where do the concepts set new precedents?
+ - Why is this feature needed?
+ - What are some example use cases?
+ - Can it be done today? Is there a workaround?
 -->
 
-### Implementation
+## Impact
 
 <!--
-How you would implement the design in Javascript?
-How you would implement the design in C++?
-What parts of the Mapbox GL ecosystem will need to change to accommodate this design?
-Are there any important edge cases?
+ - What if we do nothing? What is the impact to end-users if we don't implement this feature?
 -->