Add Thai langauge (#2903)

* Updated Thai language

* locale: remove unused translation in Thai

* locale: add Thai to settings menu and application

* locale: fix activerecord.th.yml format

config/locales/activerecord.th.yml

@@ -0,0 +1,13 @@
+---
+th:
+  activerecord:
+    errors:
+      models:
+        account:
+          attributes:
+            username:
+              invalid: only letters, numbers and underscores
+        status:
+          attributes:
+            reblog:
+              taken: of status already exists