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

templates/posts/details.rs.html

@@ -47,7 +47,7 @@
     </div>
     @if article.cover_id.is_some() {
         <div class="cover" style="background-image: url('@Html(article.cover_url(ctx.0).unwrap_or_default())')"></div>
-	<img class="hidden u-photo" src="@article.cover_url(ctx.0).unwrap_or_default()" />
+        <img class="hidden u-photo" src="@article.cover_url(ctx.0).unwrap_or_default()"/>
     }
     <article class="e-content">
         @Html(&article.content)

templates/posts/new.rs.html

@@ -1,4 +1,5 @@
 @use templates::base;
+@use templates::partials::image_select;
 @use template_utils::*;
 @use validator::{ValidationErrors, ValidationErrorsKind};
 @use std::borrow::Cow;
@@ -42,21 +43,7 @@
 
         @input!(ctx.1, license (optional text), "License", "Leave it empty to reserve all rights", form, errors, "")
 
-        <label for="cover">@i18n!(ctx.1, "Illustration")<small>@i18n!(ctx.1, "Optional")</small></label>
-        <select id="cover" name="cover">
-            <option value="none" @if form.cover.is_none() { selected }>@i18n!(ctx.1, "None")</option>
-            @for media in medias {
-                @if media.category() == MediaCategory::Image {
-                    <option value="@media.id" @if form.cover.map(|c| c == media.id).unwrap_or(false) { selected }>
-                        @if !media.alt_text.is_empty() {
-                            @media.alt_text
-                        } else {
-                            @media.content_warning.unwrap_or_default()
-                        }
-                    </option>
-                }
-            }
-        </select>
+        @:image_select(ctx, "cover", i18n!(ctx.1, "Illustration"), true, medias, form.cover)
 
         @if is_draft {
             <label for="draft">