[REF] website_slides: remove promoted slide feature, add image and channel_type

Purpose of this commit is to clean a bit channel model. Promoted slide
feature is removed as people can already choose an order for a given
channel. Moreover promoted slide will not be used when having a eLearning
display of a channel.

Image field on channels is added as it is not depending on promoted slide
anymore.

To simplify future additions a search-specific template is removed to
have a unique template to display a channel content. It will ease future
merge.

This commit is linked to task ID 1936153 and PR #30985.