Allow to 'go back' in the installer

Here's a working patch.
The styling issue of the two buttons in a row is due to the CSS rule:

body.in-maintenance .form-submit {
    display: block;
}

which I'm not sure is correct anyway. Will follow up on this.
If that's fixed, we will also need a usability review.

Regarding the CSS issue, it's not all that obvious what needs to change:
update.php uses the submit buttons without a 'form-actions' wrapper. Probably it shouldn't but that's why there is this CSS rule.
We should streamline it here and there, but which way we do needs to be decided, both UI-wise and technically.

So let's start with the usability side. When disabling this CSS rule it now looks like this:

Apart from the button size I think that this is pretty nice.

As a stop-gap measure, I'm manually overwriting the "display: block" CSS rule. Can be removed later.
Also, adding some 'alt' tags to all buttons to improve accessibility - this needs an accessibility review as well.
Might still need some simpletest adjustments because the button values have been changed. We'll see what the testbot says.
Screenshot in #3 still applies, in any case.