Always use underscores in html ids

Plus some other shifting around

bookwyrm/templates/snippets/create_status/content_warning_field.html

@@ -0,0 +1,12 @@
+{% load i18n %}
+<div class="control{% if not parent_status.content_warning and not draft.content_warning %} is-hidden{% endif %}" id="spoilers_{{ uuid }}">
+    <label class="is-sr-only" for="id_content_warning_{{ uuid }}">{% trans "Spoiler alert:" %}</label>
+    <input
+        type="text"
+        name="content_warning"
+        maxlength="255"
+        class="input"
+        id="id_content_warning_{{ uuid }}"
+        placeholder="{% trans 'Spoilers ahead!' %}"
+        value="{% firstof draft.content_warning parent_status.content_warning '' %}">
+</div>