[chore] Integrating new common-i18n project

Changes in expectation of new subscription-status component
Updated gulp task that combines i18n files
- New task also includes JSON sub-properties

Changed i18n module to risevision.common.i18n
- Stub for risevision.widget.common.translate

PR relies on https://github.com/Rise-Vision/component-subscription-status/pull/23 to be merged.

@donnapep @stulees this change should not have any immediate impact on your widgets. However, going forward we are trying to rename the i18n module to `risevision.common.i18n`. In addition, the `common-i18n` repo will host translation strings for various apps and components. As for widget specific stuff that is not in there at this point, we can gradually make the shift to move them over time, when needed.

Please review. @fjvallarino FYI. Thanks.