Addon code alignments

[FinalAngel]

• Added isort and adapted imports
• Adapted code base to align with other supported addons
• Fixes #70

General

• [x] Update changelog Adapted code base to align with other supported addons
• [x] 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].
• [x] Check that python, django, django-cms and other versions are listed correctly (add badge)
• [x] Make sure Django is not pinned in setup.py
• [x] Align gitignore files
• [x] Align setup.py
• [x] Keep the two line rule

Tests

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

Internal

• [x] Run internal checklist

[codecov-io]

Codecov Report

Merging #76 into master will increase coverage by 0.05%. The diff coverage is 89.83%.

@@            Coverage Diff             @@
##           master      #76      +/-   ##
==========================================
+ Coverage   67.18%   67.23%   +0.05%     
==========================================
  Files          65       65              
  Lines        1295     1297       +2     
  Branches       95       95              
==========================================
+ Hits          870      872       +2     
  Misses        416      416              
  Partials        9        9

Impacted Files	Coverage Δ
...otstrap4/contrib/bootstrap4_picture/cms_plugins.py	62.96% <ø> (ø)	:arrow_up:
...tstrap4_grid/migrations/0002_auto_20180709_0808.py	100% <ø> (ø)	:arrow_up:
...rap4_picture/migrations/0003_auto_20181212_1055.py	100% <ø> (ø)	:arrow_up:
...ons/0002_bootstrap4picture_use_responsive_image.py	100% <ø> (ø)	:arrow_up:
...strap4_grid/migrations/0003_migrate_column_size.py	50% <ø> (ø)	:arrow_up:
...ntrib/bootstrap4_link/migrations/0002_add_icons.py	100% <ø> (ø)	:arrow_up:
...tstrap4/contrib/bootstrap4_collapse/cms_plugins.py	92.1% <ø> (ø)	:arrow_up:
...bootstrap4_content/migrations/0002_added_figure.py	0% <0%> (ø)	:arrow_up:
...ms_bootstrap4/contrib/bootstrap4_content/models.py	0% <0%> (ø)	:arrow_up:
...otstrap4/contrib/bootstrap4_content/cms_plugins.py	0% <0%> (ø)	:arrow_up:
... and 32 more

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