new api for better membership management

[connectdotz]

Is your feature request related to a problem? Please describe.

• list the sent invites, regardless of their status, for a given thread
  this is needed for the thread owner to know who has been invited, when and if they have already rejected, to avoid unwanted or duplicate invites.
• remove a peer from the thread
  This is needed so thread owner or privileged members can remove a peer if the peer didn't voluntarily leave the thread.

Describe the solution you'd like
mabye new api like these:

• textile.threads.invites <thread-id> to list the invites sent/received for the thread, including the status (accept or reject).
• textile.threads.removePeer <thread-id> <peer-address>

Describe alternatives you've considered

• to keep track of the invites I have sent myself for each thread, but I think this should be common enough that is better to be provided by the textile API instead.

Additional context
slack thread