make connectivity and quota views translatable (#2694)

* add missing stock strings for connectivity and quota

* reword quota error

* use 'Incoming/Outgoing Messages' instead of 'Inbox/Outbox'

* just say 'Not supported by your provider.' if quota cannot be read.

- the context is already given by the headline
  'Storage on domain.org'
- the string is short and does not disturb much
  (it is a very common error)
- the string does not mention 'quota' as such,
  which will be hard to translate ('Kontingentsinformationen') -
  even if ppl as we know about the Quota extensions an such things.

there was also the idea to hide the whole section,
however, that is confusing in a multi-device-usage
when things are sometimes shown and sometimes not.

src/quota.rs

@@ -128,7 +128,7 @@ impl Context {
             let folders = get_watched_folders(self).await;
             get_unique_quota_roots_and_usage(folders, imap).await
         } else {
-            Err(anyhow!("Quota not supported by your provider."))
+            Err(anyhow!(stock_str::not_supported_by_provider(self).await))
         };
 
         if let Ok(quota) = &quota {