[AB-333] refactoring templates to use Discord time stamp formatting

abstracto-modules/modmail/src/main/resources/en_US/modmail/closing/logging/modmail_close_header_embed_en_US.ftl

@@ -1,3 +1,4 @@
+<#include "format_instant">
 {
   <#include "user_detail">
   "title": {
@@ -7,7 +8,7 @@
   <#assign messageCount>${messageCount}</#assign>
   <#assign user><#if user??><@user_detail user=user/><#else>${userId}</#if></#assign>
   <#assign closingUserDescription><@member_user_name member=closingMember/> (${closingMember.idLong?c})</#assign>
-  <#assign startDate>${formatDate(startDate,"yyyy-MM-dd HH:mm:ss zz")}</#assign>
+  <#assign startDate><@format_instant_date_time instant=startDate/></#assign>
   <#assign duration>${fmtDuration(duration)}</#assign>
   <#assign closedSilently=silently>
   "description": "<@safe_include "close_closing_description"/>"

abstracto-modules/modmail/src/main/resources/en_US/modmail/modmail_thread_header_embed_en_US.ftl

@@ -1,9 +1,10 @@
+<#include "format_instant">
 {
   <#include "member_author">
   <@member_author member=member/>,
   <#include "modmail_color">,
   <#assign user><@member_user_name member=member/> (${member.user.id})</#assign>
-  <#assign joinDate>${member.timeJoined}</#assign>
+  <#assign joinDate><@format_instant_date_time instant=memberJoinDate/></#assign>
   <#assign roles><#list member.roles as role>${role.asMention?json_string}<#sep>,<#else><@safe_include "modmail_thread_header_no_roles"/></#list></#assign>
   "description": "<@safe_include "modmail_thread_header_embed_description"/>"
 }