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

templates/posts/details.rs.html

@@ -47,7 +47,7 @@
     </div>
     @if article.cover_id.is_some() {
         <div class="cover" style="background-image: url('@Html(article.cover_url(ctx.0).unwrap_or_default())')"></div>
-	<img class="hidden u-photo" src="@article.cover_url(ctx.0).unwrap_or_default()" />
+        <img class="hidden u-photo" src="@article.cover_url(ctx.0).unwrap_or_default()"/>
     }
     <article class="e-content">
         @Html(&article.content)