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-cli/src/search.rs

@@ -65,7 +65,9 @@ pub fn run<'a>(args: &ArgMatches<'a>, conn: &Connection) {
 }
 
 fn init<'a>(args: &ArgMatches<'a>, conn: &Connection) {
-    let path = args.value_of("path").map(|p| Path::new(p).join("search_index"))
+    let path = args
+        .value_of("path")
+        .map(|p| Path::new(p).join("search_index"))
         .unwrap_or_else(|| Path::new(&CONFIG.search_index).to_path_buf());
     let force = args.is_present("force");