When BBB is run as a single node or in a scaleout setup with a cluster
proxy in front (see https://docs.bigbluebutton.org/admin/clusterproxy.html) it
is useful to store client settings in browser localStorage instead of
sessionStorage. If localStorage is configured then the client will keep seetings
like notifications for user joining, chat etc across meetings.
It is not advisable to set the setting to `local` in a setup of multiple BBB
nodes without a cluster proxy in front of it because this would lead to
unexpected behaviour at users. The browser would store settings for each server
and for users it would look like BBB is sometimes store the settings and
sometimes not.
It adds the new setting
```yaml
public:
app:
userSettingsStorage: (session|local)
```
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