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Unify filters for a model field. Each table view has a list of filters,
one for each associated model. Every filter by default receives a list
of keywords and filters the respective model using a predefined set of
fields.

The user can override the default fields and define its own using the
operator "=".

For example, in a User filter, the query "John" will search by default
in the following fields: first_name, last_name, uuid. On the other hand,
the query "first_name=John" will search only in the field specified at
the left of the "=" operator.

Fix apyrgio/synnefo#179

Fix bugs in filters due to earlier (0.5.3-3) django-filter version.

The main issue was that django-filter could not handle inherited fields.

Fix apyrgio/synnefo#202

The current code structure was leading to circular imports of the files
that are responsible for each entity. To solve this, we must break each
of those files to smaller pieces.

The pieces that each of the files are broken into are:

* views: The functions that implement the views for each entity.
* actions: The functions that have to do with the actions on each
  entity.
* filters: The functions that have to do with filtering.
* utils: Auxiliary functions.

With the new structure, the users' views can import the vms' actions and
the vms' views can import the users' actions without worrying about
circular imports.