rename i18n/English/ArchiveOldPosts to ArchiveOldThreads to match workflow activity name

Add simple docs for Archive Old Threads and CacheEMSPrereqs

lib/WebGUI/i18n/English/Workflow_Activity_ArchiveOldThreads.pm

@@ -0,0 +1,17 @@
+package WebGUI::i18n::English::Workflow_Activity_ArchiveOldThreads;
+
+our $I18N = {
+	'topicName' => {
+		message => q|Archive Old Threads|,
+		context => q|The name of this workflow activity.|,
+		lastUpdated => 0,
+	},
+
+	'archive old threads body' => {
+		message => q|<p>This workflow activity will go through all Collaboration Systems (CS) in your site and, based on the settings of that CS, archive old posts.</p>|,
+		lastUpdated => 0,
+	},
+
+};
+
+1;