On portals, make the "selected" / "default" logic more straightforward

Summary:
Depends on D20349. Ref T13275. Currently, a default item is selected as a side effect of generating the full list of items, for absolutely no reason.

The logic to pick the currently selected item can also be separated out pretty easily.

(And fix a bug in with a weird edge case in projects.)

This doesn't really change anything, but it will probably make T12949 a bit easier to fix.

Test Plan: Viewed Home / projects / portals, clicked various links, got same default/selection behavior as before.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13275

Differential Revision: https://secure.phabricator.com/D20352