Warn the user about quota depletion

[ChristophWurst]

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

As a user I do not keep close track of my IMAP quota so I'm possibly using up all my assigned storage space without notice.

Describe the solution you'd like

The app should notify when I cross 90% quota. The notification should come through a Nextcloud notification so it shows on all devices.

Implementation

Periodically query the quota. Persist the current value. If the new value crosses 90% but the previous one was below (rising edge) send out a notification. This should ensure that the notification only happens once.

Describe alternatives you've considered

Show exceeded quota in the account actions menu. This can be done additionally.

Additional context

We will continue to rely on quota reported by the IMAP server. If there is no quota report we don't notify.

[miaulalala]

Any text suggestions? Currently I have 'You are currently using {percentage}% of your mailbox storage. Please make some space by deleting unneeded emails.'