Simplify wallpaper types and download metadata about them from the remote source.

[MatthewTighe]

We are planning to remove the wallpapers shipped with Firefox to our remote source. Because of that, there will no longer be a need to distinguish between local and remote wallpapers. The only special case will be the default wallpaper, which does not have an asset.

AC:

• remove explicit references to individual wallpapers
• remove distinction between Wallpaper.Local and Wallpaper.Remote
• the Expired and Promotional types can also be removed and will be replaced by metadata
• download metadata as JSON from the remote source and unmarshal it into wallpaper objects directly
• the above may require slight updates to the wallpaper URL, as the metadata will be shared between iOS and Android and the current path references the android path specifically. this would require updating the secret in CI as well. it may also be possible to link the metadata into a child directory on the remote host, which would probably be more straightforward

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