feat: add noticeboard API

### Additions

* added new type `NoticeboardMessageType`
* added new query `noticeboardMessages(includeExpired: Boolean = false): [NoticeboardMessageType!]!`
* added new mutation `createNoticeboardMessage(content: String!, expiration: DateTime!): NoticeboardMessageType!`
* added new mutation `editNoticeboardMessage(messageId: ID!, newContent: String = null, newExpiration: DateTime = null): NoticeboardMessageType!`
* added new mutation `deleteNoticeboardMessages(messageIds: [ID!]!): [ID!]!`
* added new mutation `deleteExpiredNoticeboardMessages: [ID!]!`
* added new subscription `noticeboardSubscription: (message: NoticeboardMessageType!, eventType: EventType!)`

Closes #479