Feature/381 timestamp fields

The Timestamp mixin class adds two fields, created_at and last_modified_at to the model that inherits from it. This is used in Project and Organisation by adding the mixin, and in ProjectUpdate by replacing the existing timestamp fields with the ones inherited from the mixin. Technically this only leads to a renaming of the existing fields in the schema migration.

The renaming of the timestamp fields for ProjectUpdate leads to a number of template edits.

Also included is a script, akvo/scripts/timestamper.py that's intended to be run once to populate the created_at field with data from the django_admin_log for objects we have data on. (Only Organisations and Projects created in the admin have that data.)