alter backend to allow insertion of new stock ids

Some more tweaking is needed, like always use a higher id than max_id at
this time.
add functionality to get next possible stock id for new stocks
reverte to not editable form in backend
Seperated Edit and New creation of stocks

Prior to this the backend did not allow the creation of new stocks
because of automatic id setting.
Merge pull request #3 from nhp/develop

Allow creation of new stock items in the admin backend