Improve tab indexing

[bennothommo]

While the fields in the Configuration area are able to be tabbed through, it would be worth making it so that tabbing the last field on each of the sections (Site, Database, Administrator) takes you to the next tab immediately.

[jaxwilko]

@bennothommo I noticed this issue too, I haven't got a fix for it but in my ui branch I added tabindex="-1" to the step-links so that tabbing off the end of the form wouldn't place make the introduction step-link active making it look like the installer just reset itself. For sure needs a proper solution though