When group submissions and blind marking are enabled, an error is thrown
when a teacher trys to download all submissions in a zip. The fix is to
move the url rewriting from the plugin to the assign class so it is
done in a standard way by all modules. The rewriting is done to make images
in a text editor field resolve correctly.
This was preventing the view full submission/feedback link from appearing
everywhere in the assignment when the submission/feedback content had been
truncated in the summary because it was too long (or too many files)
Also move the 'default' setting for each plugin into it's own settings file
so that it is handled properly on new installs.
(ensure the plugin is available at setting time)
Also make the file submission plugin on by default
