Slow page startup on older browsers

[benatwork99]

Relates to #4

Older browsers are much slower to start the app - I'm seeing times from 3-15 seconds to fully load the interface on different browsers on my work desktop, worse on older versions of IE.

We do show the loading spinner, although this freezes during the long startup delay (I imagine because a single JS function is taking a long time to complete).

We could try an improve the actual time to start further (as in task #4), and/or just try to improve the responsiveness of the app so it doesn't look like it's frozen, if this is worth improving.