Merge branch 'attach-file-cursor-position' into 'master'

Adds the markdown link at the cursor poisition

When writing a comment and then dropping an image it will correctly add the image markdown link at the cursor position

![drop-image-markdown](/uploads/52f41e38faca48c9ee01a02a75ec71be/drop-image-markdown.gif)

Tested in Chrome & Firefox and both working fine 👍 

Fixes #12822 

cc. @jschatz1 

See merge request !2648

app/assets/javascripts/dropzone_input.js.coffee

@@ -65,8 +65,7 @@ class @DropzoneInput
         return
 
       success: (header, response) ->
-        child = $(dropzone[0]).children("textarea")
-        $(child).val $(child).val() + response.link.markdown + "\n"
+        pasteText response.link.markdown
         return
 
       error: (temp, errorMessage) ->
@@ -128,6 +127,7 @@ class @DropzoneInput
       beforeSelection = $(child).val().substring 0, caretStart
       afterSelection = $(child).val().substring caretEnd, textEnd
       $(child).val beforeSelection + text + afterSelection
+      child.get(0).setSelectionRange caretStart + text.length, caretEnd + text.length
       form_textarea.trigger "input"
 
     getFilename = (e) ->