Add translator notes about how to translate "plain-ol'-vanilla bridges".

 * FIXES part of #12122.
Rearrange TRANSLATORS comment in bridges.html template so it gets extracted.

 * FIXES #12122. This string wasn't getting extracted to the PO template
   file, lib/bridgedb/i18n/templates/bridgedb.pot.
Re-extract translation PO template strings. Fixes #12122.
Merge branch 'fix/12122-translator-notes' into develop