Fix file upload handling

When using enter from tag textarea, there are two events being
triggered. First one is the global textarea and the other one is post
form specific.

The check for `@current_request` didn't cancel the event if passed and
thus allowed form submission to be handled by the browser.

Additionally, jquery `submit` function doesn't trigger non-jquery event
listener so it needs to be changed to use native browser trigger.

app/javascript/src/legacy/post-upload-form.coffee

@@ -56,6 +56,7 @@ PostUploadForm::form_submit_event = (e) ->
   if e.stopped
     return
   if @current_request?
+    e.preventDefault()
     return
   $('post-exists').hide()
   $('post-upload-error').hide()

app/javascript/src/post.coffee

@@ -44,7 +44,7 @@
     $('#post_tags').on 'keydown', (e) ->
       if e.which == 13
         e.preventDefault()
-        $('#edit-form').submit()
+        document.getElementById('edit-form').requestSubmit()
       return
     return
   return