Replace `import_object()` with Django's `import_string()`

[jeremystretch]

Proposed Changes

extras.plugins.utils defines a custom import_object() function, which is used to dynamically import modules specified by a string. This can be removed and replaced with Django's own import_string() function.

Although this is not a documented part of the plugins API, we should bind this to a minor release to avoid any surprises for plugin authors who opted to use it.

Justification

Eliminates unnecessary code and leverages a Django utility.