Editor: Update sharing container to use selected site from Redux state

[aduth]

Blocked by #751

This pull request seeks to make use of the new sites module introduced in #751 to eliminate the dependency on the sites-list site object passed to the <EditorSharingContainer /> component. It also includes the necessary wiring to ensure that the selected site is assigned in the My Sites selectedSite middleware function, and that the sites reducers are included in the global reducer.

Testing instructions:

Verify that post editor Publicize connections are accurate for the site visited, that no errors are shown in the developer tools console, and that Publicize connections update upon changing the selected site in the post editor.

1. Navigate to the Calypso post editor
2. Select a site, if prompted
3. Note that the Sharing accordion displays with a loading indicator and is replaced with the set of Publicize connections available for that site (or blank if no Publicize connections)
4. Change the selected site from the New Post button
5. Note that the Publicize connections update to reflect the connections available for the newly selected site

[jkudish]

I discovered #867 while testing this, but it turned out to be in master too (probably happened from #338?)

Otherwise, this works and looks good to me, pending #751 of course.

[aduth]

Rebased to account for #751, which was recently merged.

[gwwar]

🎉

[gwwar]

Tested on Chrome, IE11, Edge.