Chore: API to get conversations count (#808)

Addresses: #497

app/views/api/v1/accounts/conversations/meta.json.jbuilder

@@ -0,0 +1,7 @@
+json.data do
+  json.meta do
+    json.mine_count @conversations_count[:mine_count]
+    json.unassigned_count @conversations_count[:unassigned_count]
+    json.all_count @conversations_count[:all_count]
+  end
+end