## 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/property.py
import json
from typing import Any, Dict, List, Optional, Union
from django.db.models import Exists, OuterRef, Q
from posthog.constants import PROPERTIES
from .person import Person
class Property:
    key: str
    operator: Optional[str]
    value: str
    type: str
    def __init__(
        self, key: str, value: str, operator: Optional[str] = None, type: Optional[str] = None, **kwargs
    ) -> None:
        self.key = key
        self.value = value
        self.operator = operator
        self.type = type if type else "event"
    def __repr__(self):
        return "Property({}: {}{}={})".format(
            self.type, self.key, "__{}".format(self.operator) if self.operator else "", self.value,
        )
    def to_dict(self) -> Dict[str, Any]:
        return {
            "key": self.key,
            "value": self.value,
            "operator": self.operator,
            "type": self.type,
        }
    def _parse_value(self, value: Union[int, str]) -> Union[int, str, bool]: