#146 stopped a group's members sharing its mailboxes on. The same shortcut lets them through everywhere else a group owns things: a member counts as the account's owner, so Calendar/set, AddressBook/set and FileNode/set skip the share check, and so does the WebDAV ACL method. Who has what a group owns is decided by who is in the group. For a member through a group only (is_group_member_only): - Calendar/set, AddressBook/set and FileNode/set refuse a shareWith change as forbidden, on create and update; for files at the top of the account too, not only inside a folder; - the DAV ACL method answers 403 on the group's calendars, address books and files; - myRights reports mayShare false (JmapRights::owner_rights), and the DAV current-user-privilege-set leaves out all and write-acl. Reading who something is shared with is unchanged, as in JMAP. Tests: a new jmap::group_share module has a member create with a share, create without one (and check myRights), share afterwards, and an outsider reach each kind; the WebDAV ACL test has a member try the ACL method on the group's folders; the IMAP ACL test now checks #146's SETACL refusal, which had no test of its own. jmap_tests, webdav_tests and imap_tests pass (RocksDB). specs/multi-account.md MA-D0.