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/partials/image_select.rs.html

@@ -0,0 +1,25 @@
+@use template_utils::*;
+@use plume_models::medias::*;
+
+@(ctx: BaseContext, id: &str, title: String, optional: bool, medias: Vec<Media>, selected: Option<i32>)
+
+<label for="@id">
+	@title
+	@if optional {
+		<small>@i18n!(ctx.1, "Optional")</small>
+	}
+</label>
+<select id="@id" name="@id">
+    <option value="none" @if selected.is_none() { selected }>@i18n!(ctx.1, "None")</option>
+    @for media in medias {
+        @if media.category() == MediaCategory::Image {
+            <option value="@media.id" @if selected.map(|c| c == media.id).unwrap_or(false) { selected }>
+                @if !media.alt_text.is_empty() {
+                    @media.alt_text
+                } else {
+                    @media.content_warning.unwrap_or(i18n!(ctx.1, "No description"))
+                }
+            </option>
+        }
+    }
+</select>