Edit blogs, and add blog icons and banners (#460)

Also adds a parameter to `md_to_html` to only render inline elements (so that we don't have titles or images in blog descriptions). And moves the delete button for the blog on the edition page.

I still have to update the SQLite migration once others PRs with migrations will be merged.

Also, there will be a problem when you edit a blog while not owning its banner or icon: when validating they will be reset to their default values… I don't see a good solution to this until we have a better way to handle uploads with Rocket (the same is probably happening for articles btw).

And the icon/banner are not federated yet, I don't know if I should add it to this PR or if it can come after?

![image](https://user-images.githubusercontent.com/16254623/53894510-7d853300-4030-11e9-8a2c-f5c0b0c7f512.png)
![image](https://user-images.githubusercontent.com/16254623/53894539-8b3ab880-4030-11e9-8113-685a27be8d7c.png)

Fixes #453
Fixes #454

plume-models/src/follows.rs

@@ -34,14 +34,16 @@ pub struct NewFollow {
 }
 
 impl Follow {
-    insert!(follows, NewFollow, |inserted, conn| {
-        if inserted.ap_url.is_empty() {
+    insert!(
+        follows,
+        NewFollow,
+        |inserted, conn| if inserted.ap_url.is_empty() {
             inserted.ap_url = ap_url(&format!("{}/follows/{}", CONFIG.base_url, inserted.id));
             inserted.save_changes(conn).map_err(Error::from)
         } else {
             Ok(inserted)
         }
-    });
+    );
     get!(follows);
     find_by!(follows, find_by_ap_url, ap_url as &str);
 
@@ -88,7 +90,11 @@ impl Follow {
         )?;
 
         let mut accept = Accept::default();
-        let accept_id = ap_url(&format!("{}/follow/{}/accept", CONFIG.base_url.as_str(), &res.id));
+        let accept_id = ap_url(&format!(
+            "{}/follow/{}/accept",
+            CONFIG.base_url.as_str(),
+            &res.id
+        ));
         accept.object_props.set_id_string(accept_id)?;
         accept.object_props.set_to_link(from.clone().into_id())?;
         accept.object_props.set_cc_link_vec::<Id>(vec![])?;