new command: `wp bp groups update_description` and a couple enhancements

Hey Boone. Here's a command I wrote to change group descriptions. We had to change ~35 group descriptions, and there wasn't already a function, so I wrote one. Also, we had to make group activity updates on these same 35 groups, and so I added some better handling of group slugs to allow for easier programmatic group activity updates. Hope this will be useful to others!