admin: Check if an action can apply to a target

There are some cases when the UI will let the user do an action to a
target, even though the action can't apply to it.

In these cases, we must check if the action can apply to a target before
actually applying it. This commit fixes that behavior.
admin: Unify filters

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
Merge branch 'feature-admin-improvements' into feature-admin