Unify size format display

[johnbillion]

#25643 replaced a couple of buggy blocks of code with calls to size_format().

There are some other places where we can use size_format() instead of manually constructing strings with file size units in them.

Patch coming up.

[wonderboymusic]

Looks like only display_space_usage() - let's patch it up

[jacklenox]

One problem here is that get_space_allowed() (used by display_space_usage()) retrieves the space allowed in megabytes, whereas size_format() converts a given number of bytes.

So in order to get this to work, we need to multiply the given figure by 1 million (roughly), which feels a bit hackish.

Or we could change the way get_space_allowed() works, but that's tied into the mu blog option: blog_upload_space, which is also in megabytes.

[helen]

No patch, punting. johnbillion, not sure if you still have a patch lying around.

[pento]

28239.diff​ changes display_space_usage() to use size_format().

[pento]

In 45579:

Multisite: Use size_format() in display_space_usage().

Deleted code is the best code.

Fixes #28239.