Add a presentation of Plume and of the instance on the homepage

Fixes #132

plume-models/src/schema.rs

@@ -57,6 +57,8 @@ table! {
         short_description -> Text,
         long_description -> Text,
         default_license -> Text,
+        long_description_html -> Varchar,
+        short_description_html -> Varchar,
     }
 }