Improve the Grid layout style for the Take Action covers block based on UX recommendations

Tasks:

• Follow the wireframes recommendations (see screenshot).
• Rename « select posts » by « select take action pages .
• Rename "rows to display" by "items to display".
• Replace the  «select tags» field by a dropdown menu where we can choose from available tags.
• 3 items (covers) should be selected by default in the "items to display" section.
• The « Load more » button behavior should remain the same. If 3 covers are displayed, the « load more » button should load 3 more covers. If 6 covers displayed, the « load more » button should load 6 more covers.
• 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.
• Add a section to enable a "view all link".
• Description of the block: Add a characters limit to 120 characters.