## Is your feature request related to a problem?

Can't define desired message format in an Action.

## Describe the solution you'd like

Increase character limit significantly.

## Describe alternatives you've considered

none

## Additional context

200 characters are way too little, especially if you have lots of variables with long names.
A single placeholder can easily take 50 characters, esp. with the overly long `[event.properties.…]` syntax.

<img width="427" alt="image" src="https://user-images.githubusercontent.com/85242/163712432-7e153e72-4385-453d-9184-287238862a8b.png">


posthog/models/action/__init__.py
import json
from django.db import models
from django.db.models import Q
from django.db.models.signals import post_delete, post_save
from django.dispatch.dispatcher import receiver
from django.utils import timezone
from posthog.redis import get_client
class Action(models.Model):
    class Meta:
        indexes = [
            models.Index(fields=["team_id", "-updated_at"]),
        ]
    name: models.CharField = models.CharField(max_length=400, null=True, blank=True)
    team: models.ForeignKey = models.ForeignKey("Team", on_delete=models.CASCADE)
    description: models.TextField = models.TextField(blank=True, default="")
    created_at: models.DateTimeField = models.DateTimeField(auto_now_add=True, blank=True)
    created_by: models.ForeignKey = models.ForeignKey("User", on_delete=models.CASCADE, null=True, blank=True)
    deleted: models.BooleanField = models.BooleanField(default=False)
    events: models.ManyToManyField = models.ManyToManyField("Event", blank=True)
    post_to_slack: models.BooleanField = models.BooleanField(default=False)
    slack_message_format: models.CharField = models.CharField(default="", max_length=200, blank=True)
    is_calculating: models.BooleanField = models.BooleanField(default=False)
    updated_at: models.DateTimeField = models.DateTimeField(auto_now=True)
    last_calculated_at: models.DateTimeField = models.DateTimeField(default=timezone.now, blank=True)
    def __str__(self):