Enabled multiple uploads in the Web IDE

What does this MR do?

Enables multiple uploads in the Web IDE. The underlying code already existed to allow for this to work, the only change needs was the multiple attribute on the <input />

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/50405

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