Remove legacy question storage (#7)

Deletes obsolete question/answer/vote persistence and the compatibility answer endpoint. Existing databases drop the legacy tables through migration 009. Plumber replies now notify the root homeowner even when nested beneath another plumber reply. Post and reply forms prevent duplicate submissions and show progress while posting.

Reviewed-on: #7
Co-authored-by: codegirl-007 <s.raide@gmail.com>

templates/partials/_post.html

@@ -3,13 +3,14 @@
   {{if canReply .User .Root}}
     <details class="post-composer">
       <summary>Reply</summary>
-      <form class="post-form" method="post" action="/posts">
+      <form class="post-form" method="post" action="/posts"
+        data-submit-once data-submitting-label="Posting…">
         <input type="hidden" name="_csrf" value="{{.CSRF}}">
         <input type="hidden" name="parent_id" value="{{.Post.ID}}">
         <label for="reply-{{.Post.ID}}">Reply to {{.Post.AuthorName}}</label>
         <textarea id="reply-{{.Post.ID}}" name="body" rows="5" required maxlength="12000"></textarea>
         <div class="post-form-actions">
-          <button type="submit" class="btn btn-primary">Post reply</button>
+          <button type="submit" class="btn btn-primary" data-submit-button>Post reply</button>
           <button type="reset" class="btn btn-ghost"
             onclick="this.closest('details').removeAttribute('open')">Cancel</button>
         </div>