If the option is enabled, it will try to find a meta tag containing
the page description and keywords, just like we do for the page title.
It will either look for regular meta tag or OpenGraph ones.
The option is disabled by default.
Note that keywords meta tags is mostly not used.
In `configure` template, the variable associated with this setting
is `$retrieve_description`.
Fixes#1302
* requires PHP 5.6
* use blazy on linklist since a lot more thumbs are retrieved
* thumbnails can be disabled
* thumbs size is now 120x120
* thumbs are now cropped to fit the expected size
Fixes#345#425#487#543#588#590
