Don't hardcode personal group name prefix, use
models.user.PERSONAL_GROUP_NAME_PREFIX instead.
Added askbot's Group model to admin, and improved list views for Space
and Feed models.
Show id in admin list view for Site model.
Show spaces in Thread admin list view, and allow keyword searching
within admin list views for Thread and Post.
Ask form submit button text now names current Site/Group if applicable.
Added __unicode__ methods to AuthUserGroups and GroupMembership models
(helps when debugging).
Merge pull request #237 from knowledgepoint-devs/modal

Fixed text value for submit button on ask form, added __unicode__ methods for AuthUserGroups and GroupMembership models, plus more improvements to admin site.