[BUG] Remove small flash

[daviddavo]

When you change the selected DAO on the dropdown, after everything is finished, it makes a little flash.

This can happen because it enters the "global" load state just a fraction of a second.

It happens just when the URL is changing.

[daviddavo]

When the url changes, the display_page callback is called. As there is no change, it will raise a PreventUpdate exception. Nevertheless, the loading state is still entered, so the loading screen will be shown a few miliseconds until the PreventUpdate exception is raised.

We could either change all out callbacks system, or use Plotly Dash Pages #88 included in v2.5.0

[daviddavo]

I split the callback into two. One that is called when the url changes, and one that modifies the layout. The one that modifies the layout is only called when the first part of the url (the platform) changes, so there won't be any visual glitches.

The code is now a bit more clean too