robot edit new group / user by name, instead of sort by id

This aims to fix occasional robot failures such as:
```
06:14:06 Create Edit Group :: Tests group creation                             | FAIL |
06:14:06 Page should have contained element 'xpath=//input[@id='id_name'][@value='test_group_1418364571']' but did not
``` 
E.g. https://ci.openmicroscopy.org/view/Failing/job/OMERO-5.1-merge-robotframework/104/console

Latest group / user is selected from the table by name (instead of sorting by ID and selecting first) which should be a bit more robust.

To test, check latest
https://ci.openmicroscopy.org/view/Failing/job/OMERO-5.1-merge-robotframework/lastCompletedBuild/console