Add a way to specify filenames when working with CSVs

[justinanderson]

• Allow users to specify a filename when exporting a table to CSV.
• Don't overwrite existing CSVs without first getting user approval.
• Auto-rename uploaded CSVs to not have conflicting filenames (with a preview of what the name will be, if possible).
• Allow users to rename existing CSVs.

[RogerTangos]

Allow users to specify a filename when exporting a table to CSV implemented in 14c8b28