Merge branch 'dropdown-enter-select' into 'master'

Fixed issue with enter key selecting wrong option in dropdown

If you search for a label and press enter the dropdown will select the last element rather than the first. This is because the currentIndex is -1 rather than 0

![labels](/uploads/eb29924109947fd63fea32e21471f05a/labels.gif)

Closes #17630

See merge request !4210

app/assets/javascripts/gl_dropdown.js.coffee

@@ -512,7 +512,7 @@ class GitLabDropdown
         return false
 
       if currentKeyCode is 13
-        @selectRowAtIndex currentIndex
+        @selectRowAtIndex if currentIndex < 0 then 0 else currentIndex
 
   removeArrayKeyEvent: ->
     $('body').off 'keydown'