Ability to add custom languages without having to create a commit request....

[MappingKat]

Steps to reproduce the error

A partner wants to add a local language to the XLS forms, but Cadasta does not allow custom languages to be uploaded. Right now, the languages must be located in https://github.com/Cadasta/cadasta-platform/blob/bf0625d56f998be0eb06aca93adad429241a2516/cadasta/config/settings/languages.py

Actual behavior

When a partner uploads a form with a custom language, the form does not upload.

Expected behavior

When a partner uploads a form with a custom language, the language dropdown uses the langauge specificed in the form and then allows the translated labels to show.