Some improvements and polishment

I've polished this feature a little bit while I am using in OA 1.0 1.2. Fixes #783360: Linking Invoices to Groups: Possible?, #1069758: Linking to groups not working, #1197270: CSS not loading and Includes some improvements such as "Add invoice" context link, strongarm settings, block to see group's latest invoices and "My invoices" block for dashboard, posting invoice into a group, etc. So, maybe useful for anyone else. Apply both patches.