Add managesieve (Sieve) support for remote accounts

When switching identities, the managesieve plugin still connected to
the default sieve server. Hook into managesieve_connect to redirect
the sieve connection to the remote account's server.

Adds sieve_host, sieve_port and sieve_auth columns to the database,
a Sieve section to the identity settings form, preconfig support for
sieve_host, and localized labels for all 7 languages.

SQL/postgres/2026021001.sql

@@ -0,0 +1,13 @@
+ALTER TABLE ident_switch
+	ADD sieve_host varchar(64);
+
+ALTER TABLE ident_switch
+	ADD sieve_port
+		integer
+		CHECK(sieve_port > 0 AND sieve_port <= 65535);
+
+ALTER TABLE ident_switch
+	ADD sieve_auth
+		smallint
+		NOT NULL
+		DEFAULT(1);