[rss] Convert old table format to new

Creates the new format table on setup. If it's empty, checks for the old
format table and adds any feeds and articles into the new one.

The old tables are then dropped. Note that this makes it impossible for
any other module to use a table named "rss" or "recent". I'd suggest
standardizing DB table names in modules to be prepended with the module
name, so this isn't an issue.