Reintroduce submodule for securimage library

Commit d36a79528391872403b4becb1ec8f21099beeb38 damaged it, and it was
subsequently removed by 8058018a86bbb64efdf62846573c78abbec8ddb5.

This revert went against the team's decision to use submodules, and
furthermore did not take into consideration bug fixes with .htaccess
files, so with this I'm reverting the revert

The web.config file added in d36a79528391872403b4becb1ec8f21099beeb38
was committed in the securimage repository, and the submodule updated to
match the new branch head.