Export: Improve usability - timing expectations, install button (#1474)

Because export might take a couple minutes, we now set expectations
that it might take 'several minutes' instead of just 'please wait.'

We also promote 'Install new Signal Desktop' from a text link in the
instructions to a button. This is important on the 'Completed' screen
because it is bigger and to the left of the 'Export Again' button, which
previously drew primary focus on that screen.

Lastly, we also remove the title-specific element of the support link,
so we're resilient to title changes in the future.

FREEBIE

background.html

@@ -14,6 +14,9 @@
       {{ /hideProgress }}
       <div class='message'>{{& message }}</div>
       <div>
+        {{ #installButton }}
+          <button class='install grey'>{{ installButton }}</button>
+        {{ /installButton }}
         {{ #exportButton }}
           <button class='export grey'>{{ exportButton }}</button>
         {{ /exportButton }}