Add support (configuration) for disabling (hiding) the ability to add new entries

ATM the "Add entry" button is always displayed, but we need to support two important use cases:

• read-only live data sources (we should be able to hide the "Add entry" button from the live data configuration, but we need to see where to put this new configuration)
• live data sources where not every user can add new entries (the "Add entry" button should be displayed only for the users that are allowed to add new entries)

We could use the same live data configuration for both use cases (i.e. the live data widget shouldn't care why the "Add entry" button needs to be hidden).