Rename variable for clarity
Add Content-Length header

This is particularly important in preventing partial content getting
cached if the connection drops mid-request.
Remove option to disable gzip serving behaviour

It was needless extra configuration: there's no good reason to disable
this and if someone really wants to they can create a sub-class and
override the `find_gzipped_alternatives` method.
Move immutable file handling into WhiteNoise base

Of course it's still up to the Django sub-class to identify which files
are immutable, but we want as much of the behaviour as possible in base
so it's available to other potential sub-classes.
Refactoring