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/users.rs

@@ -205,8 +205,8 @@ impl User {
                     public_domain: inst.clone(),
                     local: false,
                     // We don't really care about all the following for remote instances
-                    long_description: String::new(),
-                    short_description: String::new(),
+                    long_description: SafeString::new(&<String>::new()),
+                    short_description: SafeString::new(&<String>::new()),
                     default_license: String::new(),
                     open_registrations: true,
                     short_description_html: String::new(),