Attribute external code better

[marteinn]

This library uses two modules from https://github.com/django-import-export/django-import-export and currently we only attribute the project with a link in the header. This should be improved by adding a LICENSE in the file header information about git commit id (to simplify maintenance).

The two modules are:

• https://github.com/Frojd/wagtail-redirect-importer/blob/develop/wagtail_redirect_importer/tmp_storages.py
• https://github.com/Frojd/wagtail-redirect-importer/blob/develop/wagtail_redirect_importer/base_formats.py