LPS-39405 Wrong permission checking in add panel

Hey Julio,

Initial conditions to display the Add Content and Add Application tabs are the same. In both cases the user adds an application to a page. Specific conditions (e.g. view permissions, add to page permissions, create content permissions) are checked within the panel logic. These initial conditions are:

- Current state is not maximized
- Layout type is portlet
- Page hasn't a link enabled to a page template
- User has permission to update pages or this page is customizable and user has permission to customize

Conditions to add a new page are:

- User is not editing a Page Template
- User has permission to add pages

Besides, the whole add panel won't be displayed in control panel nor in live environment, if staging is enabled.

These changes solve LPS-39405 and LPS-38040.

Thanks!

@JorgeFerrer