Exclude folders from stats

[devmount]

Is your feature request related to a problem? Please describe.

As @SolidTux mentioned in #389, it is sometimes useful to exclude certain folders from the stats processing at all.

Describe the solution you'd like

Having an additional add-on setting, where a user can multi-select none, one or more folders for a given account. After reprocessing the data, the selected folders and their content won't be part of the calculated stats. If a selected folder doesn't exist anymore, it will be removed from the selection by ThirdStats.

Describe alternatives you've considered

It's also possible to implement multi-folder-selection in the filter on the stats page. But this isn't a permanent solution, since the filter has to be set anew every time.

Additional context

#389