Tag list bugfix

- renamed ContentBaseDecorator.tag_list to tags_list
  It cause error on edit tags (it override ActsAsTaggableOn's tag_list)
  in some places like expose_decorated(:content_plan, attributes: :content_plan_params)

- added rake task which corrects bad tag_lists for ContentPlan and Content
- added running of rake task in db migration
Merge pull request #67 from bitzesty/edit_tags_bugfix

Tag list bugfix
Update README: Note about db replacement
Tweak Readme
Code style updates
These flaky specs need investigation
Merge pull request #69 from alphagov/code-style-fixes

Code style fixes & mark flaky tests as pending while we investigate.