Add forward and back buttons to the main page

[mbramson]

It would nice to have some way to traverse weeks (forward and backwards) on the main page that displays pairs.

Going back is definitely useful so that you can comment on previous pairs when it isn't the current week (without knowing the route to do this off the top of your head) as per #32.

Going forward will be useful to be able to specify a user you'd like to pair with for a coming week for #34.