support template apps

[kevodwyer]

Motivation:
There is a class of App that isn't currently catered for.

• An App may share the same html/css but require a different name (customisation)
• A number of identical Apps may require a different audience (Chat channel members)

Description:
This change introduces the support for a new type of App called a Template App
A template App has an associated Chat channel

• A template app can be installed from a folder or from the recommended-apps page
• When installed, the user is prompted to enter a name for the App (format: Template - description), ie Forum - Latest News, Album - Wedding 2020.
• The existing update mechanism has been updated to support Template Apps
• A template App can be shared by inviting members as per existing Chat functionality.
• Invitations to join appear on the Newsfeed as per existing functionality.