added header message containing information about the user the mod mail thread has been opened for

abstracto-application/abstracto-modules/modmail/modmail-impl/src/main/resources/templates/thread_header/modmail_thread_header_embed_en_US.ftl

@@ -0,0 +1,16 @@
+{
+  "author": {
+    "name": "${threadUser.member.effectiveName}",
+    "avatar":  "${threadUser.member.user.effectiveAvatarUrl}"
+  },
+  "color" : {
+    "r": 200,
+    "g": 0,
+    "b": 255
+  },
+  <#assign user>${threadUser.member.effectiveName}#${threadUser.member.user.discriminator} (${threadUser.member.user.id})</#assign>
+  <#assign joinDate>${threadUser.member.timeJoined}</#assign>
+  <#assign roles><#list threadUser.member.roles as role>${role.asMention}<#sep>,<#else>No roles</#list></#assign>
+  <#assign threadCount>3</#assign>
+  "description": "<#include "modmail_thread_header_embed_description">"
+}