[GraphQL] Implement basic group querying in the Graph

Problem/Motivation

Groups are a fundemental part of our communities, we want to open this data up towards consumers of our GraphQL endpoint.

It should be possible to

• Select a group by its UUID
• Find a group by a specific property of data (e.g. it's group_type)
• Search for a topic in a similar manner as on /search/groups
• Paginate through results according to the Relay connection specification

Querying and searching for data should respect existing privacy measures (such as visibility settings) according to platform configuration.

Proposed resolution

Implement the required GraphQL Schema and DataProducer plugins in social_group that extend the schema defined on social_graphql

Remaining tasks

• Implement filtering based on specific properties
• Implement freeform search
• Implement per-field access checks based on platform configuration

User interface changes

None

API changes

The following GraphQL Query endpoints become available.

query {
  groups(...PaginationFilter!, ...?)
  group(uuid: ID!)
}

Data model changes

None