Track posts I need to follow up on

Sometimes I want to follow up on a conversation in the future, but in order to do that I have to use some kind of system outside of o2, like bookmarking it in a particular folder, or adding it to Trello/Remember the Milk/etc.

I think it'd be useful if o2 had a `Needs Follow Up` button at the bottom of every post. If I click it, that post is added to my personal list of posts that need to be followed-up on.

Then, there'd be some way to display their user's list, similar to the ToDo widget. Maybe that could work like this:

1. When a page is loaded, o2 checks if the user has any posts on the current site marked for follow-up
1. If they do, o2 automatically adds a widget to the site's sidebar showing the user their personal list of posts that need follow-up
1. Users who don't have any posts marked would not see the widget. Or maybe it'd be better UX if they did see it, along with a message saying there wasn't anything to follow up on?

In a Multisite network, I think it'd be helpful if the list would also show posts on other sites that need follow up. That way I don't have to visit every o2 that I follow in order to see all my posts that need follow-up. Maybe the widget would have two tabs: 1) Posts on the current site; and 2) Posts on all sites. The first tab would be displayed by default.

(similar to the ToDo widget

Start by reading the existing ToDo widget and the post-bottom and site-sidebar entry points described in the issue, including how page loading and WordPress Multisite are handled. Define the follow-up data scope and UI behavior before implementation; done means users can mark posts, view their personal current-site and all-sites lists, and see the intended empty-state behavior.

Written by the indexing model from the issue text.