Enable setting of inviter for invitee reward

This reward is currently disabled on mobile, but if we wanted to enable it, we should have a way of setting the inviter. Most people who go through invites will do this on lbry.tv though and claim it there.

Start by tracing how mobile invites and invitee rewards are currently handled, then compare that flow with claiming on lbry.tv. Define how the inviter would be identified and set before attempting to enable the reward on mobile; completion should include a clear implementation path and validation of the reward flow.

Written by the indexing model from the issue text.