make blog/instance description a SafeString

long_description & short_description's documentation say they can be
Markdown, but they are String, not SafeString.

This led to escaped strings being printed in the editor
https://github.com/Plume-org/Plume/issues/220

plume-models/src/instance.rs

@@ -3,6 +3,7 @@ use diesel::{self, QueryDsl, RunQueryDsl, ExpressionMethods, PgConnection};
 use std::iter::Iterator;
 
 use plume_common::utils::md_to_html;
+use safe_string::SafeString;
 use ap_url;
 use users::User;
 use schema::{instances, users};
@@ -16,8 +17,8 @@ pub struct Instance {
     pub blocked: bool,
     pub creation_date: NaiveDateTime,
     pub open_registrations: bool,
-    pub short_description: String,
-    pub long_description: String,
+    pub short_description: SafeString,
+    pub long_description: SafeString,
     pub default_license : String,
     pub long_description_html: String,
     pub short_description_html: String
@@ -30,8 +31,8 @@ pub struct NewInstance {
     pub name: String,
     pub local: bool,
     pub open_registrations: bool,
-    pub short_description: String,
-    pub long_description: String,
+    pub short_description: SafeString,
+    pub long_description: SafeString,
     pub default_license : String,
     pub long_description_html: String,
     pub short_description_html: String