Display descriptions of datasets

Depends on #71 (closed)

• As an economist
• I want to read the description of a dataset in the search results or on the dataset page
• in order to better understand what it is about.

Acceptance criteria

• The description of each dataset having a description MUST be displayed in the dataset page in the definitions list.
• On the dataset page, the description MUST be displayed after the "provided by..." mention, and before the matching series.