Add a continuous delivery and release workflow. The continuous delivery
workflow is used to build the Docker image for FeedDeck and the release
workflow is used to generate the changelog.
Instead of rendering the images for a Nitter item within the description
in the details view, we are now rendering the images via the
"ItemMediaGallery" widget. To not render the images twice, we also added
the "disableImages" paramter to the "ItemDescription" widget, to not
render the images in the 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.
