add a workaround for a broken 1.19

http://jehy.ru/articles/2012/09/17/mediawiki-1-19-unable-to-save-thumbnail-to-destination/
also lockdown edit on non confirmed users
install does not work if the users table doesn't exist
allow setting of plain_options
add a few default options for extensions. they can later be overwritten with plain_options statement
correct ending
fix typo
passwords might contain special chars
make it possible to set a wiki to readonly mode