Admin feature: add a bunch of pretty_name methods to all the BAC subclasses

[prioux]

Some of the BAC subclasses have a method pretty_name() to help display what the BAC is about in the index page. E.g. BackgroundActivity::RegisterFile will show "Register #{cnt} files on #{source_name}".

The default pretty_name method just runs demodulize.underscore.humanize on the class name.

This issue is about making the description of all BAC subclasses better.

In particular, I'd like the RubyRunner class to show the first line of comment (if any) of the "process_item()" method provided by the admin. That way when the admins configures many many RubyRunner for the daya to day maintenance, they can tell which BAC does what.

[prioux]

For RubyRunner, this has been done in 4ab8f67