Sorting sorting tasks 919857

Created by: birdsarah

This pull request implements sorting including reverse sorting. Things to note:

• There are a set of default sort orders, that Tom specified, these are set in views.py e.g. the first time you click on id it will be reverse sort, the first time you click on excerpt it'll be asc sort
• When you click again on the same header the sort order is then reversed
• The tests have been fixed so that mysql and sqllite now test fine
• I have snuck in Hamish's request for pagination at the bottom
• The icon for sorting has been deleted as am using style unicode character.