Make `gnome-shell-extension-prefs` the main way that extensions are managed

Currently, Software allows people to install extensions, view which ones they have installed, and also remove them. It doesn't do a fantastic job at this, and it's another type of software for it to have to care about.

As a result, we've talked about removing the extension support from Software and adding some of the required functionality to gnome-shell-extension-prefs instead.

The rough idea is:

• Software continues to handle updating extensions that are packaged (essentially treating them as part of the OS)
• gnome-shell-extension-prefs becomes the "Extensions" app, which must be installed for people to add their own extensions
• There will be no extension installation in the Extensions app - they must be installed from extensions.gnome.org
• The new Extensions app will allow extensions to be removed and updated

There are some initial mockups for what the Extensions app could look like, here.

The UX advantage of this would be that we'd remove some bad and potentially confusing UI from Software, and have a nicer way to update and manage extensions for those who care about them.