Addon code alignments

[FinalAngel]

• Added isort and adapted imports
• Adapted code base to align with other supported addons

General

• Update changelog
  Adapted code base to align with other supported addons
• Add contributors text to README file
  We're grateful to all contributors who have helped create and maintain this package. Contributors are listed at [link to https://github.com/divio/djangocms-video/graphs/contributors].
• Check that python, django, django-cms and other versions are listed correctly (add badge)
• Make sure Django is not pinned in setup.py
• Align gitignore files
• Align setup.py
• Keep the two line rule

Tests

• Make sure naming for the test files is test_.py
• Add tests_migration.py file if missing
• Remove test migration check in other files
• Check requirements.txt file for accuracy (e.g. unpin djangocms-helper)
• Check settings.py that it is ordered correctly
• Added isort and updated files

Internal

• Run internal checklist

[codecov-io]

Codecov Report

Merging #39 into master will increase coverage by 0.08%.
The diff coverage is 100%.

@@ Coverage Diff @@ ## master #39 +/- ## ========================================== + Coverage 84.39% 84.48% +0.08%  ========================================== Files 13 13 Lines 173 174 +1 Branches 10 10 ========================================== + Hits 146 147 +1  Misses 26 26 Partials 1 1

Impacted Files	Coverage Δ
...angocms_file/migrations/0011_auto_20181211_0357.py	100% <ø> (ø)	⬆️
djangocms_file/migrations/0001_initial.py	100% <100%> (ø)	⬆️
djangocms_file/cms_plugins.py	82.6% <100%> (ø)	⬆️
djangocms_file/migrations/0008_add_folder.py	100% <100%> (ø)	⬆️
djangocms_file/models.py	73.23% <100%> (+0.38%)	⬆️
djangocms_file/migrations/0006_migrate_to_filer.py	73.33% <100%> (ø)	⬆️
djangocms_file/migrations/0007_adapted_fields.py	100% <100%> (ø)	⬆️

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