Implement preview for comments

Adds a "Preview" button alongside "Comment". Clicking this button
renders the comment text without actually creating the comment.

https://todo.sr.ht/~sircmpwn/todo.sr.ht/195

scss/main.scss

@@ -213,9 +213,19 @@ select.form-control {
     top: -0.1rem;
   }
 
-  .event > blockquote {
-    margin-top: 0.5rem;
-    border: none;
+  .event {
+    > blockquote {
+      margin-top: 0.5rem;
+      border: none;
+    }
+    &.preview {
+      border: 1px dashed black;
+      .preview-tag {
+        float: right;
+        font-size: 0.8rem;
+        font-style: italic;
+      }
+    }
   }
 }
 

todosrht/blueprints/ticket.py

@@ -6,7 +6,7 @@ from srht.database import db
 from srht.oauth import current_user, loginrequired
 from srht.validation import Validation
 from todosrht.access import get_tracker, get_ticket
-from todosrht.filters import invalidate_markup_cache
+from todosrht.filters import invalidate_markup_cache, render_markup
 from todosrht.search import find_usernames
 from todosrht.tickets import add_comment, mark_seen, assign, unassign
 from todosrht.tickets import get_participant_for_user
@@ -138,6 +138,7 @@ def ticket_comment_POST(owner, name, ticket_id):
     resolve = valid.optional("resolve")
     resolution = valid.optional("resolution")
     reopen = valid.optional("reopen")
+    preview = valid.optional("preview")
 
     valid.expect(not text or 3 <= len(text) <= 16384,
             "Comment must be between 3 and 16384 characters.", field="comment")
@@ -159,6 +160,14 @@ def ticket_comment_POST(owner, name, ticket_id):
         ctx = get_ticket_context(ticket, tracker, access)
         return render_template("ticket.html", **ctx, **valid.kwargs)
 
+    if preview == "true":
+        ctx = get_ticket_context(ticket, tracker, access)
+        ctx.update({
+            "comment": text,
+            "rendered_preview": render_markup(tracker, text),
+        })
+        return render_template("ticket.html", **ctx)
+
     participant = get_participant_for_user(current_user)
     event = add_comment(participant, ticket,
         text=text, resolve=resolve, resolution=resolution, reopen=reopen)

todosrht/templates/ticket.html

@@ -321,6 +321,17 @@
       </div>
       {% endif %}
       {% endfor %}
+
+      {% if rendered_preview %}
+      <div class="event preview">
+        <span class="preview-tag">Comment preview</span>
+        <a href="{{ current_user|user_url }}">{{ current_user }}</a>
+        <blockquote>
+          {{ rendered_preview }}
+        </blockquote>
+      </div>
+      {% endif %}
+
       {% if TicketAccess.comment in access %}
       {% if current_user %}
       <form
@@ -370,6 +381,12 @@
           {% endif %}
           {% endfor %}
         </select>
+        <button
+          type="submit"
+          name="preview"
+          value="true"
+          class="btn btn-default pull-right"
+        >Preview {{icon("caret-right")}}</button>
         {% else %}
         <button
           type="submit"