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:
1. Open an existing issue or merge request with comments.
2. Move a file over the new comment dropzone. One paperclip icon should appear.
3. Now click the edit button for an existing comment and repeat step 2. You should see two paperclips.
Fixes #1725.
See merge request !959
