Made following changes to admin screens:
* added language column and filter to list views for User, Thread and
Post models.
* added column to Post list view showing the first 30 chars of the
Post's text
* added site column and filter to list view for Thread model
* general tidying (including removing debug print statements and moving
filters with long lookup lists to bottom of list of filters)
Merge pull request #295 from knowledgepoint-devs/merge1

Made following changes to admin screens: