Mention button pre-fill user's handle once

• Bug location: Pleroma-FE
• Pleroma-FE version: b225c357
• Steps to reproduce:
  1. go on a profile
  2. click on Mention button
  3. write post and send it (post has user's handle pre-filled
  4. click again on Mention button
  5. username (@foobar@example.com) is not pre-filled

If page is refreshed, user's handle is inserted again when clicking on Mention button.