Build hooks to allow Amplify actions to be recorded as Relationships per-user

[AramZS]

[boonebgorges]

The only Amplify actions shipped in PF are: 'Send to Draft' and 'Tweet'.

For 'Send to Draft' (which is really 'Nominate' - I'm not sure why this language is used), we could introduce a simple do_action() into Nominated::build_nomination().

'Tweet' - and any other buttons that a plugin might add using 'pf_amplify_buttons' - don't actually result in requests to the server. So I think what we'd need to add is a mechanism that sends an AJAX ping to WP that says "an Amplify button has been pressed" and then fire an appropriate WP hook.

I don't love the idea of building this sort of developer-facing feature without having a specific use for it, either in PF or in a partner's customizations. One possibility is that we could introduce a 'Tweeted by' feature into PF. When a user clicks 'Tweet', a tweeted_by relationship entry is saved; then, we indicate somewhere in the Reader/post.php interface that an item has been tweeted by that user. The problem here is that clicking 'Tweet' doesn't actually create a tweet, it just opens the window. So perhaps we'll hold off on this feature until we have a clearer use case.