Improve the media handling within the app. We do not save the media
files for items to the Supabase storage anymore. The source icons are
now only saved in the Supabase storage, the usage of an url as source
icon is not possible anymore.
The media files for items are directly retrieved from the corresponding
url or for the web version from the "image-proxy-v1" Supabase function.
If the image is retireved from the Supabase function we cache the image
in the browser via the cache control headers. If the image is directly
retrieved from it's url it's cached by the "CachedNetworkImage" widget.
It is now possible to use a Nitter instance, where the RSS endpoints are
protected via basic authentication. For that a new
"FEEDDECK_SOURCE_NITTER_BASIC_AUTH" environment variable was added,
which can be used to set the value for the authentication header.
