## Bug description

The following filter fails with a 500 internal error.

![2021-02-04_12-07](https://user-images.githubusercontent.com/148820/106877644-a9f4ce80-66e1-11eb-8320-87bb213bdde2.png)


## Expected behavior

Error is shown to user about invalid regular expression.

Note no errors shown on the page as well.

## How to reproduce

1. Go to events
2. Add filter "Current URL matches regular expression ?*"
3. See infinite loading screen / error

## Environment

- [x] PostHog Cloud
- [x] self-hosted PostHog, version/commit: _please provide_

## Additional context

Note this can also happen when filtering by events.

#### *Thank you* for your bug report – we love squashing them!

posthog/models/filters/test/test_filter.py
def test_filter_by_all(self):
        filter = Filter(
            data={
                "properties": [
                    {
                        "key": "name",
                        "value": json.dumps({"first_name": "Mary", "last_name": "Smith"}),
                        "type": "person",
                    }
                ],
                "date_from": "all",
            }
        )
        date_filter_query = filter.date_filter_Q
        self.assertEqual(date_filter_query, Q())
    def test_default_filter_by_date_from(self):