Fix #4770: Allow flaggers to update flag reason.

app/views/post_flags/_reasons.html.erb

@@ -2,6 +2,10 @@
   <li>
     <span class="prose"><%= format_text(flag.reason, inline: true) %></span>
 
+    <% if policy(flag).edit? %>
+      (<%= link_to "edit", edit_post_flag_url(flag) %>)
+    <% end %>
+
     <% if policy(flag).can_view_flagger? %>
       (<%= link_to_user(flag.creator) %>, <%= time_ago_in_words_tagged(flag.created_at) %>)
     <% else %>

app/views/post_flags/edit.html.erb

@@ -0,0 +1,15 @@
+<div id="c-post-flags">
+  <div id="a-edit">
+    <h1>Edit Flag</h1>
+
+    <p>
+      Editing flag for <%= link_to "post ##{@post_flag.post_id}", @post_flag.post %>.
+    </p>
+
+    <%= edit_form_for(@post_flag) do |f| %>
+      <%= f.input :reason, as: :dtext, inline: true %>
+      <%= f.button :submit, "Submit" %>
+      <%= dtext_preview_button "post_flag_reason" %>
+    <% end %>
+  </div>
+</div>