Turn the admin filter_* functions into methods

These `filter_file` and `filter_folder` functions make assumptions about
the user model in use -- it must have `username`, `first_name`, and
`last_name` fields. They should be implemented as methods instead of
inner functions to allow overriding them when using a different user
model.

Another solution for searching custom user models could be to
dynamically build the query based on `get_user_model()._meta.fields`.
This seems complicated and prone to error, so providing overridable
methods is simpler and probably good enough.
Merge pull request #322 from fusionbox/filter_methods

Turn the admin filter_* functions into methods