Update the public field from Dashboard to be a JSON

[jefmoura]

Purpose

We need to provide a possibility to share a dashboard in different ways at the same time.

Approach

• Removed old public field and added a new one to store the combined information of sharing
• Updated the dashboard's signal to use the new field

Furter Info

@Menda I need to update the database manually before merging this change into production and demo, otherwise, we're going to lose all sharing info of old dashboards.

Related ticket: https://github.com/Humanitec/ActivityAPI/issues/459

[jefmoura]

Travis is green but it's not updated on GitHub.