Allow editors to export the submissions table to a CSV file

[NateWr]

Describe the problem you would like to solve
Editors often use third-party spreadsheet software to help them manage their editorial load. These editors want a way to export the submissions to a CSV file so it can be imported to a spreadsheet.

Describe the solution you'd like
Add a button above the submissions table to export submissions.

Who is asking for this feature?
This was a recommendation after user tests for #7495.

Additional information
The following mockup places the export button next to the filters button.