Topic/permission management via API

[aepace]

Users developing LIGO follow-up pipelines are requesting the ability to manage topics and permissions via the client API. In particular:

1. list_topics lists the topics for which there is already a permission added via the web interface. There should be the ability to list topics are available to add permissions. So, topics that are available to individuals or group members.

2. Add the ability to add and remove permissions via the api.

3. List the available permissions for each topic via the API.

[cnweaver]

This all makes good sense. In terms of adding and removing permissions, I assume this related both to managing the permissions attached to a credential, and managing the permissions delegated to other groups, and we will want to support both.