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

src/routes/posts.rs

@@ -263,6 +263,7 @@ pub fn update(
                 &Instance::get_local(&conn)
                     .expect("posts::update: Error getting local instance")
                     .public_domain,
+                false,
             );
 
             // update publication date if when this article is no longer a draft
@@ -422,6 +423,7 @@ pub fn create(
             &Instance::get_local(&conn)
                 .expect("post::create: local instance error")
                 .public_domain,
+            false,
         );
 
         let searcher = rockets.searcher;