Allow alternate mount points for image renditions

[fluffy-critter]

Expected Behavior

Replace/extend the image_output_subdir with an option which can take an alternate output path that is not a subdirectory of the static content directory, and optionally enable an alternate mount path for the output directory.

Context

Some deployment environments want the entire application directory to be read-only, and being able to write out the content to an alternate location (which could then either be mounted by the Flask instance or by the fronting server) would help to facilitate this.

It is a low priority but it has been requested.