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/posts.rs

@@ -210,6 +210,7 @@ impl<'a> Provider<(&'a Connection, &'a Worker, &'a Searcher, Option<i32>)> for P
         let (content, mentions, hashtags) = md_to_html(
             query.source.clone().unwrap_or_default().clone().as_ref(),
             domain,
+            false,
         );
 
         let author = User::get(
@@ -756,7 +757,7 @@ impl Post {
             post.license = license;
         }
 
-        let mut txt_hashtags = md_to_html(&post.source, "")
+        let mut txt_hashtags = md_to_html(&post.source, "", false)
             .2
             .into_iter()
             .map(|s| s.to_camel_case())
@@ -994,7 +995,7 @@ impl<'a> FromActivity<LicensedArticle, (&'a Connection, &'a Searcher)> for Post
             }
 
             // save mentions and tags
-            let mut hashtags = md_to_html(&post.source, "")
+            let mut hashtags = md_to_html(&post.source, "", false)
                 .2
                 .into_iter()
                 .map(|s| s.to_camel_case())