WebUI: provide legend text

It provides semantic meanings for the option group.

src/webui/www/.htmlvalidate.json

@@ -5,7 +5,6 @@
   "rules": {
     "long-title": "off",
     "no-inline-style": "off",
-    "prefer-button": "off",
-    "wcag/h71": "off"
+    "prefer-button": "off"
   }
 }

src/webui/www/private/download.html

@@ -22,6 +22,7 @@
             <textarea id="urls" rows="10" name="urls"></textarea>
             <p>QBT_TR(Only one link per line)QBT_TR[CONTEXT=HttpServer]</p>
             <fieldset class="settings" style="border: 0; text-align: left; margin-top: 6px;">
+                <legend>QBT_TR(Torrent options)QBT_TR[CONTEXT=AddNewTorrentDialog]</legend>
                 <table style="margin: auto;">
                     <tbody>
                         <tr>

src/webui/www/private/upload.html

@@ -18,6 +18,7 @@
             <input type="file" id="fileselect" accept=".torrent, application/x-bittorrent" name="fileselect[]" multiple>
         </div>
         <fieldset class="settings" style="border: 0; text-align: left; margin-top: 12px;">
+            <legend>QBT_TR(Torrent options)QBT_TR[CONTEXT=AddNewTorrentDialog]</legend>
             <table style="margin: auto;">
                 <tbody>
                     <tr>