Show the remaining SMS balance on the alarms page

Retrieve the remaining balance on the Nexmo account and show it on the
alarm page. This seems like a good place, as it should be instantly
visible when enabling the alarm that the SMS notification won't work (if
the balance it too low).

Note that the request is currently synchronous. If it turns out it slows
the page down too much it should be possible to make an asynchronous
request for it (through jQuery).
Show the balance in red when it gets too low.

Consider less than 2 euro left as being too low, although that still
means we can send quite a lot of text messages (price is 0.06 euro per
text right now).

While here also right-align the status text and the remaining balance.
Try to make it easier to change the default front page.

Name the light groups page 'lightgroups' instead of just 'index' and
introduce a separate 'front' page (which just calls the lightgroups page
for now).
Separate URL for lightgroups

Use 'front' as the default landing page, give lightgroups its own path.
Make alarms page the default landing page

As requested by Natalie.
Make the footer show the current section.

I.e. highlight power if we're on the power page, alarm if we're on the
alarms page, ...