Add Notification Topic Groups

[ChrisPenner]

Overview

• Run migration

Adds notification topic groups so end users can subscribe to groups of topic semantically, e.g. "Watch Project" or "Subscribe to this ticket and its updates", but we can still expand these sets of topics the groups correspond to.

Implementation notes

• Leaves the existing individual topic list so things like webhooks can still be specific about which events they want
• Adds a list of subscribed topic groups to each subscription
• Adds a topic group for watching everything in a project

Test coverage

• Transcripts

[ChrisPenner]

@hojberg creating subscriptions now requires both topics and topicGroups; but either can be empty.