[UX] Add a log-in form to the update.php page, and expose it to anonymous users ONLY when db updates are pending.

Describe your issue or idea

If updates are available, and you are not logged into your site, we should place a log-in form on the update.php page so admins don't need to go elsewhere, log in, and then come back to run updates.

We shouldn't show the log-in form here all the time, so that we aren't creating a back door to the site, but when updates are pending it would be useful to provide one.

Background: this idea/suggestion came up during today's dev meeting (unfortunately Youtube was acting up, so the meeting could not be recorded). The trigger behind this was the problem of the "catastrophic" error message people will be getting when they upgrade their sites from 1.9.x and earlier versions, to the upcoming 1.10.0 version. And the cause of that in turn is the changes coming with #225. When this happens, people will need to run update.php in order to recover their site, and they cannot do that unless they have logged in before (or if they happen to be advanced users using drush). To account for the rest of the people facing this problem now and in the future, we thought to consider this as an option.