fixup! Bug 7494: Create local home page for TBB.

Bug 43131: Reduce layout jank when loading about:tor.

We wait until the initialization is complete before we reveal the page
content. So on refresh, the page just shows a single flash, but no
change in layout.

We also speed up the usual page load by having l10n load both the stable
and testing headings at initiation, rather than waiting for the
"InitialData" event from the page actor.

In the case where we have an update to show, we wait a little longer for
the l10n to complete.