Handle filename conflicts during exports

[eloquence]

If a file filename.extension already exists in an export, a filename filename(1).extension should be assigned, with the number incremented as required. It's important that the number is before the extension, to not break file handling that is dependent on the extension (e.g., for DOCX files).

So, for example:

benign artichoke/secret_document.pdf
benign artichoke/secret_document(1).pdf
benign artichoke/secret_document(2).pdf
glued trajectory/trustme.exe

In terms of avoiding race conditions during numbering, we can assume that files are exported sequentially, i.e. the user can not kick off a new export operation until a previous one has been completed.

[eloquence]

Currently filename conflicts are handled via the subdirectory structure documented here:
https://github.com/freedomofpress/securedrop-export#export-folder-structure

We may need to reconsider other conflict resolution strategies if/when we revisit the export format; closing for now.