Add post and image deletion

This one was kind of a doozy. This also adds a custom 403 error page and
fixes some permission denied behavior that I was having issues with for
a while.

This is also set up in a way that hopefully will allow me to easily
implement user post deletion.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

board/templates/403.html

@@ -0,0 +1,8 @@
+{% extends "board/base.html" %}
+{% block title %}
+Permission denied
+{% endblock title %}
+
+{% block content %}
+<h3>You do not have permission to do this.</h3>
+{% endblock content %}

board/templates/board/post_confirm_delete.html

@@ -0,0 +1,23 @@
+{% extends "board/base.html" %}
+{% load i18n %}
+
+{% block content %}
+<form id="form" method="post" action="{% url 'board:post_delete' post.id %}">
+    {% csrf_token %}
+    <table>
+    <tr>
+        <td><input type="submit" value="{% translate 'Delete post' %}" /></td>
+        <td><input id="delete_image" type="submit" value="{% translate 'Delete image only' %}" /></td>
+    </tr>
+    </table>
+    {{form.image_only}}
+</form>
+
+<script>
+$(document).on("click","#delete_image", (e) => {
+    e.preventDefault();
+    $("#{{form.image_only.auto_id}}").val(1);
+    $("#form").submit();
+});
+</script>
+{% endblock content %}

board/templates/board/post_delete_success.html

@@ -0,0 +1,33 @@
+{% extends "board/base.html" %}
+{% load i18n static %}
+{# Title #}
+{% block title %}{% translate "Post delete success" %}{% endblock %}
+{# Body #}
+{% block content %}
+<div class="row" id="message">
+    {# We do not use pluralize filter for "seconds" because it's a pain to get it to translate. #}
+    {% blocktranslate %}Delete successful. This window will close in {{window_timeout}} second(s).{% endblocktranslate %}
+</div>
+
+<script>
+function isIframe() {
+    try {
+        return window.self !== window.top;
+    } catch (_) {
+        return true;
+    }
+}
+
+setTimeout(function() {
+    if(isIframe()) {
+        let deleteWindow = getDeleteWindow();
+        if(deleteWindow) {
+            deleteWindow.closeFrame();
+        }
+    } else {
+        window.close();
+    }
+}, 1000 * {{window_timeout}});
+
+</script>
+{% endblock %}

board/templates/board/post_snippet.html

@@ -4,6 +4,7 @@
     id="p{{post.id}}"
     class="post"
     data-report-url="{% url 'board:report_form' board.url post.id %}"
+    data-delete-url="{% url 'board:post_delete' post.id %}"
     {% if perms.board.add_ban %}
     data-ban-url="{% url 'board:ban_create' board.url post.id %}"
     {% endif %}