tables.sql: Improve description of old_flags

tables.sql: Improve description of old_flags

* Changed the name of the 'utf8' flag to 'utf-8', as that is what
  Revision stores. (This was already corrected in the mediawiki.org
  manual page by RichF.) Noted that 'utf8' was, however, mistakenly
  used in an old version of recompressTracked.php.
* Added 'external' to the list of flags. This was already added
  to the manual page (by GreenReaper) yet not here. Copied the
  description from there and added a couple of clarifications.

Change-Id: If15b49a28d7d4b0397481d21e30d877298fc7955