Add a "Check for updates" button to the Updates tab for manual repo refresh

1. Right now the only way to manually check for updates is "swipe to refresh" on the Latest tab, which is not very discoverable. That's okay, since most people will just be using automatic repo updates, but it would be nice if it were more discoverable.

2. The empty screen on the Updates tab reads, "Congratulations! All of your apps are up to date (or your repositories are our of date)." However, it does not say how to actually update your repositories.

An elegant way to address both of these issues is adding an "Update repositories" button on the updates page. I'm imagining it just like the "Find people near me" button on the Nearby page.

It would also be nice to add the timestamp of the last check for updates (but maybe I should make a separate issue for this, since it does not depend on adding a refresh button).