Prevent '/' in workspace/scope/... names

Allowing / in names will create issues with URLs, but is currently allowed:

$ ./manage.py create_workspace 'a/b'
$ ./manage.py list_workspaces
Name     Public    Default Expiration Delay (days)    Default File Store      # Other File Stores
-------  --------  ---------------------------------  --------------------  ---------------------
System   False     Never                              Default (Local)                           0
a/b      False     Never                              Default (Local)                           0

$ ./manage.py scope create 'a/b'
$ ./manage.py dbshell
debusine=> SELECT * FROM db_scope;
 id |    name    
----+------------
  1 | debusine
  2 | localscope
  3 | a/b