Fix failing db:populate because of space logos

db:populate won't create logos for spaces anymore.
Was failing because the folder default_space_logos was removed.
Fix empty password inputs when editing models, refs #949

It was actually the expected behavior to have an empty value in a password
input. But in these inputs we actually want to show the values, so we have
to explicitly set it when creating the input.
This will be valid for inputs of the type 'showable_password'. For standard
password inputs the value won't be displayed (and they shouldn't).
Better submenu for admin pages in spaces, refs #459