Implement "Empty Junk" for the Junk folder context menu. b=189911 r+sr=bienvenu

suite/locales/en-US/chrome/mailnews/messenger.dtd

@@ -534,6 +534,8 @@
 <!ENTITY folderContextCompact.accesskey "F">
 <!ENTITY folderContextEmptyTrash.label "Empty Trash">
 <!ENTITY folderContextEmptyTrash.accesskey "y">
+<!ENTITY folderContextEmptyJunk.label "Empty Junk">
+<!ENTITY folderContextEmptyJunk.accesskey "J">
 <!ENTITY folderContextSendUnsentMessages.label "Send Unsent Messages">
 <!ENTITY folderContextSendUnsentMessages.accesskey "d">
 <!ENTITY folderContextUnsubscribe.label "Unsubscribe">

suite/locales/en-US/chrome/mailnews/messenger.properties

@@ -381,3 +381,8 @@ confirmPhishingUrl2=%1$S thinks this site is suspicious! It may be trying to imp
 
 #LOCALIZATION NOTE %1$S is the e-mail address of the person we will allow remote content for
 alwaysLoadRemoteContentForSender=Click here to always load remote content from %1$S.
+
+# mailCommands.js
+emptyJunkTitle=Confirm
+emptyJunkMessage=Are you sure you want to permanently delete all messages and subfolders in the Junk folder?
+emptyJunkDontAsk=Don't ask me again.