Feature: import and export content items

[rvalitov]

Ability to save or load content items of the widget. This will help to quickly move or copy a widget from one site to another.

• export items
• import items
• import + export integration, tests

The functionality of this feature is similar to #32 . The difference is that here we export content items only, not the settings of the widget. But technically the approach is the same and probably these 2 issues can be merged into one in the future.