feat: warn before duplicate mobile issue capture (#371)

frontend/index.html

@@ -355,6 +355,12 @@
         <div class="small" id="create-issue-label-status" aria-live="polite">Choose a repository to load labels.</div>
         <div class="create-issue-label-list" id="create-issue-label-list"></div>
       </fieldset>
+      <section class="create-issue-duplicates" id="create-issue-duplicates" aria-live="polite" hidden>
+        <strong>Possible existing issues</strong>
+        <div id="create-issue-duplicate-status" class="small"></div>
+        <div id="create-issue-duplicate-list" class="create-issue-duplicate-list"></div>
+        <button id="create-issue-anyway" type="button" hidden>Create anyway</button>
+      </section>
       <div class="small">The issue will be assigned to you.</div>
       <div class="create-issue-actions">
         <button id="save-unfiled-issue" type="button">Save for filing</button>