Force storage.file_overwrite to True

[Gwildor]

This fixes uploading a static file with the same name twice resulting in
a second file with a randomized suffix in the name, and of course the
first one never to be updated to the newer version.

This is a bit related to #30 and basically has the same fix. It's a bit
surprising that this never was a problem before, but that's probably
because, in contrary to the setting from #30, that default in
django-storages for this setting is the one we need. This also means
that the impact of this change is a lot smaller. It only affects
projects which have manually set this setting to False, most likely to
prevent user uploads from overwriting each other.

[antonagestam]

I'm reluctant to merging this, simply because I don't think it's a great thing to be overwriting user settings. I don't really see the value added, since if you deliberately add file_overwrite = False to your settings I don't see how you could be surprised by the behaviour that it creates.

Having Collectfast changing these settings means that maintaining and having stable releases becomes harder since more code depends on other libraries.