Restructure converter code to better support plugin-supplied converters

Two related changes:

  * Removed the static importer list from the import_job schema.  Now each
    converter can supply its own list of supported import options, and the
    frontend will query for them at runtime.

  * Moved the LCNAF converter rules into the plugin directory and arranged for
    it to take advantage of the new pluggable behaviour.
Move deep_merge utility into asutils
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