Task form UI fix

Aligned 'assign users' selectbox
Merge pull request #68 from bitzesty/task_form_ui_corrects

Task form UI fix
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