Add deletion of progress updates

And only show progress edit for editing unfinished readthroughs

bookwyrm/templates/snippets/readthrough_form.html

@@ -7,6 +7,8 @@
         <input type="date" name="start_date" class="input" id="id_start_date-{{ readthrough.id }}" value="{{ readthrough.start_date | date:"Y-m-d" }}">
     </label>
 </div>
+{# Only show progress for editing existing readthroughs #}
+{% if readthrough.id and not readthrough.finish_date %}
 <div class="columns">
     <div class="column">
         <div class="field">
@@ -29,6 +31,7 @@
         </div>
     </div>
 </div>
+{% endif %}
 <div class="field">
     <label class="label">
         Finished reading