UI for moderator dashboard

[lpatmo]

We need to design the UI for what the moderator will see when they log in to start manually matching cards.

[lpatmo]

Proposal:

Moderators will see a moderator-only "Moderators" link in the nav. When they click on it, they'll be taken to the /moderators screen, where they'll see unmatched entries they can match.

And a list of top-flagged users.

[angelocordon]

@lpatmo @distalx feel free to flesh this area out a bit more; happy to help figure out a design that makes sense to keep our UI consistent (see #34, for example)