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/instance.rs

@@ -128,8 +128,8 @@ impl Instance {
         short_description: SafeString,
         long_description: SafeString,
     ) -> Result<()> {
-        let (sd, _, _) = md_to_html(short_description.as_ref(), &self.public_domain);
-        let (ld, _, _) = md_to_html(long_description.as_ref(), &self.public_domain);
+        let (sd, _, _) = md_to_html(short_description.as_ref(), &self.public_domain, true);
+        let (ld, _, _) = md_to_html(long_description.as_ref(), &self.public_domain, false);
         diesel::update(self)
             .set((
                 instances::name.eq(name),