FE for "Uploads to a wiki should be stored inside the wiki git repository"

What does this MR do?

This is based off of !21362 (merged) and addresses the comments (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21362#note_97361621).

• Updates dropzone_input to lookup uploads_path from a data attribute rather than a global variable. Currently this variable is being set more than once, which causes a dependency on execution order.
• Fixes a bug in dropzone_input where the entire responseText is shown when the API returns a JSON error message.
• Minor refactoring to dropzone_input to make it testable.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides