Update 'Cards' blog post layout to use Likes instead of Ratings

The objective of this ticket is to replace the display of the ratings average by the number of likes on the Cards blog post layout. See a mockup below:

What encouraged this change is that :

• The Cards blog post layout introduced a dependency between the blog application and the ratings application, so when the blog application is installed the ratings is also installed.
  This is problematic on recent versions of XWiki because when the ratings application is installed, the ratings are enabled on all pages by default and require the user to disable them manually in the wiki Administration if it's not needed (see BLOG-161).
• Since the refactoring of Ratings in recent versions of XWiki (12.9RC1+) the ratings are not displayed anymore in the Blog cards layout (see BLOG-176). Note that, the usage of the ratings on the Cards layout is not documented.
• On recent versions of XWiki the Like feature is enabled by default on all pages, so, having in addition the ratings feature used on blog posts is redundant and could confuse the users

So, what need to be done on this ticket is :

• Get rid of the dependency of the blog application to ratings application
• On the Cards blog post layout replace the ratings stars by the likes heart with the number of likes
• Update the Cards layout documentation on the Blog documentation