Add Mailing lists to Administration

A mailing list is an address that passes mail on to everyone on it. To
Stalwart it is its own object, x:MailingList, behind sysMailingList*, so
it gets its own section under Directory after Groups: search, fifty to a
page with each list's recipient count, and a panel to create, edit and
delete one.

Recipients are a property of the list, so unlike a group's members they
save with the rest of the panel. What Save sends for them is only what was
added and removed, one recipients/<address> pointer each -- the patch the
live server accepted -- so a recipient added elsewhere while the panel was
open is not taken out. They can be pasted several at a time, from a
spreadsheet column, a comma-separated line or Name <address>; anything with
an @ that is not an address stays in the box with a note. Past a dozen, a
filter narrows them.

That is all a list is in Stalwart -- no owners, moderation or posting
rules -- so that is all the panel offers.

The mock answers x:MailingList with two lists, the recipient set's live
shape, and the refusals a wrong address, a clash with an account and a
missing permission get.

Twenty-five new strings and one plural, in all nine catalogues.

KNOWN-ISSUES.md

@@ -71,6 +71,8 @@ works the same way — and dropped where 0.15 was the whole subject. Support for
   - **Membership is the member's.** `"memberGroupIds/<group>": true` on the user was accepted; `{"@type": "User", "memberGroupIds": <group>}` then found them with a total of 1, and the user's own `memberGroupIds` read `{"<group>": true}`. The same pointer with `null` took them out again and left the set as it was before.
   - **A group with members cannot be deleted**: `objectIsLinked`, with `objectId` as `{"object": "Account", "id": <group>}` and `linkedObjects` listing each member as `{"object": "Account", "id": …}`. With the member out, the delete went through and the group read back as `notFound`.
 
+  The same run tried a throwaway mailing list, `[email protected]`, before the Mailing lists section was written. It was created with `recipients` as a set, `{"[email protected]": true}`, and read back as `name`, `domainId`, `description`, `aliases`, `memberTenantId`, `recipients` and a computed `emailAddress`. `"recipients/<address>": true` added one and left the other; a `text` filter found it; it was destroyed with nothing linked. Not tried: removing a recipient with `null` (the same set patch as a group membership, which was), and how the server words a recipient that is not an address.
+
   Still from source only: that membership gives a member no permissions (`access_token.rs` builds a user's permissions from their own roles), and that groups cannot nest.
 
 - **A refused password shows the server's reason in English.** Every other refusal from the registry is said in the reader's language: each error type has its own message, and a value one of Stalwart's validators refused — a domain name, an address, an empty field — is recognised by the validator's wording and explained again rather than shown. A password policy is the exception, on purpose. Its rule is the server's to set, so there is nothing to translate it from in advance, and its reason follows a translated sentence rather than being dropped, which would leave "not accepted" with no way to find out why.