Brief flickering of rendering artifacts while loading the UI

A really tiny UI glitch: when loading the TinyPilot UI (e.g. http://tinypilot.local), the browser might very briefly display some intermediate artifacts while initializing the frontend:

You can observe this effect in the screen cast video of this PR, around the 2:01 time mark.

To me, it appears as if these buttons come from our dialogs: e.g., the first button row looks like the ones from the shutdown dialog (without labels, though), the button underneath is probably a dangling “close” button.

The root cause might be that the dialogs are statically dumped into the index.html page. So the artifact flickering could stem from the fact that there is a short time gap between the browser initially rendering the dialogs’ HTML, and then hiding the dialogs in a later step.

This issue might be browser-dependent.

Reproduce the loading sequence at http://tinypilot.local and inspect app/templates/index.html around the statically included dialogs. Then compare those elements with app/templates/custom-elements/shutdown-dialog.html and the later dialog-hiding step. Done means the UI no longer briefly shows dialog or button artifacts during initialization, including in affected browsers.