Fixes for "max upload size" value showed in the UI

This fix brings the following:
1) Update the "max upload size" value after scanFiles() is done and also after file uploads to reflect a more actual value
2) The "max upload size" aka storage statistics are now shown based on which directory the user is currently in, which can change between "user storage", "external storage" and "shared storage of another user"

Note that for the "shared storage" case I need to look into why it's not using the quota stream wrapper, which I'll address in a separate PR.

Please review @karlitschek @icewind1991 @DeepDiver1975