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/mysql/2026021001.sql

@@ -0,0 +1,15 @@
+ALTER TABLE `ident_switch`
+	ADD `sieve_host` varchar(64) AFTER `smtp_auth`;
+
+ALTER TABLE `ident_switch`
+	ADD `sieve_port`
+		int
+		CHECK(`sieve_port` > 0 AND `sieve_port` <= 65535)
+		AFTER `sieve_host`;
+
+ALTER TABLE `ident_switch`
+	ADD `sieve_auth`
+		smallint
+		NOT NULL
+		DEFAULT 1
+		AFTER `sieve_port`;