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

static/css/_forms.scss

@@ -129,3 +129,7 @@ form.new-post {
   input[type="submit"]:hover { background: $lightgray; }
   -webkit-appearance: none;
 }
+
+.button + .button {
+  margin-left: 1em;
+}

static/css/_global.scss

@@ -137,6 +137,10 @@ p.error {
   margin: 0px;
 }
 
+.user .avatar.medium {
+  margin-left: 0px;
+}
+
 .badge {
  	margin-right: 1em;
  	padding: 0.35em 1em;