Resolve "Attaching multiple files to a wiki page doesn't work"

What does this MR do?

Previously, when trying to attach multiple files to a wiki page simultaneously, we got an error thrown: 9:Could not update master. Please refresh and try again" error. This was a result of the race condition on the backend when we did upload multiple files in parallel.

The proposed solution was to make files upload sequentially. To achieve this, we added a config parameter to our dropzoneInput function so we could pass a number to parallelUploads setting of the Dropzone.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides

Issue: #197927 (closed)