Add some checks for TableRule definition in admin

[antisvin]

There's no validation at the moment.

1. It should run from_yaml/from_json loading, to make sure that rule text is correct
2. It should check that condtions can be parsed (currently they can be checked by trivial regex).

[miraculixx]

From my POV that's not going to do it. Rather this should be refactoring as described in this comment