Fixes #15374

Summary of the issue:
At present, if you use the available add-ons category, add-ons already installed are not shown in the list.
This is causing confusion, where users expect add-ons to listed in the available add-ons tab, regardless if installed.

The main use case is the following user story:
I am a user who has installed VeryCoolAddon. I want to recommend a friend to use it. I check in the add-on store via the "Add-on catalog" tab (formerly "Available add-ons" tab) that the add-on is still in the store and its version. Note: add-ons may be removed from the store for various reasons, so you cannot be certain if the add-on is still available in the add-on store if it is already installed.

Description of user facing changes
add-ons which are already installed will no longer be hidden in the available add-ons list

Description of development approach
Include all the same statuses in the installed add-ons tab in the available add-ons tab. (#15374)