Remove uses of dropzone jquery wrapper
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## Description
The following discussion from gitlab-ce!21466 should be addressed:
- [ ] @pslaughter started a [discussion](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21466#note_99092246): (+5 comments)
> We ought to stop using the jQuery wrapper entirely at some point... probably too big for the scope of this MR though. Let's open a gitlab-ce~3011693 issue
## Proposal
Let's just use `new Dropzone`!
issue