Add Quota to Connectivity View (#2612)

* add imap::get_quota_roots()

* schedule quote-checking job on getting connectivity-html

* get quota and debug print it

* basic quota output

* update quota at most once per minute, emit event on changes

* use more meaningful names

* add some comments, move update_recent_quota() to quota.rs

* show root name only if there are several roots

* make clippy happy, some refactorings

* allow only one update-quota job per time

* add now supported QUOTA to standards.md

standards.md

@@ -10,6 +10,7 @@ Text and Quote encoding          | Fixed, Flowed ([RFC 3676](https://tools.ietf.
 Filename encoding                | Encoded Words ([RFC 2047](https://tools.ietf.org/html/rfc2047)), Encoded Word Extensions ([RFC 2231](https://tools.ietf.org/html/rfc2231))
 Identify server folders          | IMAP LIST Extension ([RFC 6154](https://tools.ietf.org/html/rfc6154))
 Push                             | IMAP IDLE ([RFC 2177](https://tools.ietf.org/html/rfc2177))
+Quota                            | IMAP QUOTA extension ([RFC 2087](https://tools.ietf.org/html/rfc2087))
 Authorization                    | OAuth2 ([RFC 6749](https://tools.ietf.org/html/rfc6749))
 End-to-end encryption            | [Autocrypt Level 1](https://autocrypt.org/level1.html), OpenPGP ([RFC 4880](https://tools.ietf.org/html/rfc4880)), Security Multiparts for MIME ([RFC 1847](https://tools.ietf.org/html/rfc1847)) and [“Mixed Up” Encryption repairing](https://tools.ietf.org/id/draft-dkg-openpgp-pgpmime-message-mangling-00.html)
 Configuration assistance         | [Autoconfigure](https://developer.mozilla.org/en-US/docs/Mozilla/Thunderbird/Autoconfiguration) and [Autodiscover](https://technet.microsoft.com/library/bb124251(v=exchg.150).aspx)