Create awareness that user may need to increase memory limit

[eteubert]

Only affects the import path.

When uploading the audio file, the receiving WordPress instance needs to be able to hold the whole file in RAM before saving it. Some files may be too large for default configurations. The workaround is:

# wp-config.php
define( 'WP_MEMORY_LIMIT', '512M' );

Currently the imports just fail if WP runs out of memory. The user should be made aware of this workaround, possibly even before starting the import if we detect large (>60MB?) files in the feed.