Allow deleting TrustAtoms

[harlantwood]

Agent can delete TA's which they authored (ie they are the DHT-level author of the Link under the TA).

Specifically, the agent can delete:

• all of my TAs with a given target

Maybe later, in a future story, the agent can delete:

• all of my TAs with a given target and a given full content
• all of my TAs with a given full content

[harlantwood]

Needed by MewsFeed for “unfollow”. Mentioned in https://github.com/GeekGene/MewsFeed/issues/94

[harlantwood]

Closed by #75