Don't send ephemeral message unless app is in channel

[dennissivia]

When trying to send an (ephemeral) message to slack and the app is not installed on that channel, slack responds with a no_permissionerror.
In order to avoid this error and by extension reduce the noise in our exception tracking, we should skip payloads with is_app_in_channel: False.

Example sentry error payload:

{
  api_app_id: [Filtered], 
  authed_teams: [
    [Filtered]
  ], 
  event: {
    channel: [Filtered], 
    is_app_in_channel: False, 
    links: [{}], 
    [...]
    type: link_shared, 
    [...]
  },
  type: event_callback
}

Error response:

{
  code: slackclient_platform_error, 
  data: {
    error: no_permission, 
    ok: False
  }
}