bugfix: image widget: make sure source is present

fixes #286
bugfix: image widget: make sure source is present
bugfix: blog: newer posts link should not be clickable

fixes #287
bugfix: editbar: move down content only when editing is active
update changelog
bugfix: video widget: use correct mime type for video example
update gems in test app
Merge branch 'develop' into feature/diagram-widget

* develop:
  update gems in test app
  bugfix: video widget: use correct mime type for video example
  update changelog
  bugfix: editbar: move down content only when editing is active
  bugfix: blog: newer posts link should not be clickable
  bugfix: image widget: make sure source is present
  bugfix: image widget: make sure source is present
some refactoring and cleanup