Make convertUserOptions.php more self-contained

* Moved the code from User::decodeOptions() to the maintenance script,
  the only place it is (indirectly) used.
* Changed the script to insert new rows itself rather than calling
  User::saveSettings(), avoiding bug 63677.
* Removed FOR UPDATE and COMMIT lacking a matching BEGIN.
* Made ORDER BY explicit.
* Removed pointless "Do each user sequentially [...]" comment dating
  back to r48732, in which the script would increment a user's ID
  to get the next one.

Bug: 63677
Change-Id: I86365a7363af3376cc4f55fa528b050b44378656