SES-1740 Workbench warns on existing id in create-memory-rdfs-dt now.
Merge branch 'master' into SES-1740
SES-1740 All create-* pages now use an additional request to check if the id already exists.

* If so, a warning allows the user to back out.
* create-federate is exempted because its validation precludes entering an existing id.
Merged in dvisser/sesame/SES-1740 (pull request #79)

SES-1740 All create-* pages now use an additional request to check if the id already exists.