Make the upload location configurable (default "svg")

[VanL]

The default location for media uploads was "media", which put the location as MEDIA_ROOT/media (usually /media/media/file.svg). This patch does the following:

• Makes the default directory configurable (override the class variable in a subclass)
• Makes the default directory "svg" (so "file.svg" ends up by default in MEDIA_ROOT/svg/file.svg)
• Allows the storage location to be overridden in a subclass (so as to upload to /favicons instead of MEDIA_ROOT, for example)

[Aleksi44]

Thanks @VanL :+1:

Deployed in version 0.0.13