excel

https://www.pivotaltracker.com/story/show/75664656

Adds MS Excel (XLS, XLSX) file support to MCM
they're already encoded as unicode, but with `&` and other characters that may break when we try to check the string length to see if it'll fit into our DB CharField `max_length` constraint. See here for where it was breaking: https://github.com/buildingenergy/seed-core/blob/feature/75664656-excel-xls-import/seed/models.py#L1197