Notification Hub: add additional filters

[knolleary]

The initial iteration of the platform-wide announcement feature allows an admin to target users based on their role.

We have some additional requirements to be able to target different cohorts of users. This issue is to capture the additional filters required so we can prioritise their implementation:

• By Team Type - ie send to Enterprise Team owners only
• By Billing state - ie send to teams with active subscription (exclude trial, cancelled, suspended teams)

The filters will be relatively quick to add; the api added in #4637 has room for additional filter flags to be added.