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

migrations/postgres/2019-03-06-115158_blog_images/down.sql

@@ -0,0 +1,4 @@
+-- This file should undo anything in `up.sql`
+ALTER TABLE blogs DROP COLUMN summary_html;
+ALTER TABLE blogs DROP COLUMN icon_id;
+ALTER TABLE blogs DROP COLUMN banner_id;

migrations/postgres/2019-03-06-115158_blog_images/up.sql

@@ -0,0 +1,4 @@
+-- Your SQL goes here
+ALTER TABLE blogs ADD COLUMN summary_html TEXT NOT NULL DEFAULT '';
+ALTER TABLE blogs ADD COLUMN icon_id INTEGER REFERENCES medias(id) ON DELETE SET NULL DEFAULT NULL;
+ALTER TABLE blogs ADD COLUMN banner_id INTEGER REFERENCES medias(id) ON DELETE SET NULL DEFAULT NULL;