[IMP] website_slides: allow to set cover image for webpage slide

Before this commit, the user can only create a webpage slide
by setting its name, category and tags. On the channel grid,
no tumbnail was shown.
This commit allow the user to upload a cover image from the
slide upload modal and to modify it from form view. the
mimetype of webpage slide is now 'forced' through the
modal to 'text/html' to avoid conflit with 'image/png'
when setting the cover image.

Task-1902304

0 files changed, 0 insertions, 0 deletions