Create a new carousel layout style for the Take Action covers block based on UX recommendations and the usability testing results

Tasks:

• Follow the wireframes recommendations (see screenshots).
• We should add a « layout » section with the Carousel & Grid options. The Carousel option should be selected by default when users create a new block.
• The "rows to display" section should be hidden when the Carousel layout is selected.
• Replace the  « select tags » field by a dropdown menu where we can choose from available tags. This way users don’t have to remember which tags exist.  
• We could display a placeholder image instead of an error message when the content block is empty, so that users could quickly visualize how the block looks like even without content.

• We should set  by default 12 items maximum for the carousel layout. This should not be editable.
• Add a section to enable a "view all link". 
• Description of the block: Add a characters limit to 120 characters.