Django-stdimage

* Remove Django 3.0 DeprecationWarning 215

* Fix 206 -- Overwrite variations by defaults 213

Test Heroku CI

* Add `JPEGField` that behaves like the `StdImageField` but all variations are JPEGs.
* Add `delete_orphans` argument to remove orphaned files.
* Remove `pre_delete_delete_callback` signal receiver in favor of `delete_orphans` argument.
* Remove `pre_save_delete_callback ` signal receiver in favor of `delete_orphans` argument.

* Add Serbian Cyrillic and Serbian Latin translations (207)
* Include compiled messages in dist packages (208)
* Update package setup and distribution

* Fix 203 -- Prevent access to deferred fields in stdimage file descriptor (204)