Do not add the same icon multiple times to dropzones
Some DOM queries were to broad which caused paperclip icons and spinners to be added multiple times to a dropzone in case of multiple dropzones on a page.
Steps to reproduce:
- Open an existing issue or merge request with comments.
- Move a file over the new comment dropzone. One paperclip icon should appear.
- Now click the edit button for an existing comment and repeat step 2. You should see two paperclips.
Fixes #1725 (closed).