Control sensitive fields better

[LightOfHeaven1994]

This is going to help us avoid backend failures with 500 as we didn't check what user put into some fields and python had issues with it. Example, when user put into group field incorrect value "component=foo", as the result backend fails as Run object doesn't have this attribute:

  File "/app/ibutsu_server/filters.py", line 84, in string_to_column
    column = getattr(model, field)
AttributeError: type object 'Run' has no attribute 'component=foo'

If there are some issues to find a column - we return None from string_to_column helper function. Then widgets that have this field mandatory will return empty data