[Web] Fixes to Sieve validation (fixes #1027)

data/web/inc/lib/sieve/extensions/fileinto.xml

@@ -3,6 +3,7 @@
 <extension name="fileinto">
 
 	<command name="fileinto">
+    <parameter type="tag" name="create" regex="create" occurrence="optional" />
 		<parameter type="string" name="folder" />
 	</command>
 

data/web/inc/lib/sieve/extensions/mailbox.xml

@@ -0,0 +1,8 @@
+<?xml version='1.0' standalone='yes'?>
+
+<extension name="mailbox">
+
+	<test name="mailboxexists">
+    <parameter type="string" name="folder" />
+	</test>
+</extension>